Well, before I can give my opinion on that question, I think I have to ask a different one.
Will the Void Century flashback JUST be about the Void Century/Second World? Or will we also be learning about the First World at the same time?
Because, for these bigger flashbacks, Oda has kind of been bundling different subjects together, recently. Combining the "main" subject for the flashback with another topic that he covers at the same time.
With the Oden flashback, we didn't JUST get a story about Oden. We also basically got Roger's flashback at the same time. Along with the reveal about Laugh Tale.
With Bonney's flashback, we didn't JUST get Bonney's backstory. We also got Kuma's backstory, and a bit of Vegapunk's, and even a taste of God Valley, all at the same time.
With the Harald and Loki flahsback, we didn't JUST learn about how Loki killed Harald. We also got the entire story of God Valley in detail as well as more Roger flashback.
So, I think it's pretty likely, whenever we actually do get the Void Century flashback, we'll probably also get ANOTHER flashback within that flashback. Probably centering on the First World, but possibly covering something else, who knows?
So...instantly that makes it possible for this to be the longest flashback in One Piece history. Covering TWO of the biggest mystery eras in the entire series, all at the same time? Introducing Oda-only-knows how many side-characters, as Oda loves to do? Especially since there's been NO setup for the most of the characters outside of Joyboy, Nika, Imu, and Davy Jones. You KNOW Oda's going to give each of those guys their own "crews" of a dozen side-characters each. Setting up stuff for the Ancient Weapons, Haki, Haki Knots, Poneglyphs, the origin of Devil Fruits, and the VERY GODS THEMSELVES?! Yeah...yeah...that's going to take some time.
Will it actually be longer than God Valley/Loki's flashback? I'm not so sure. But, the potential IS there, I have to admit that.
But, there are a few things that could cut down on that length a little.
One is Oda's health. Oda's not getting any younger, and probably not much healthier. It feels like the longer he makes this, the more he risks just never being able to finish it. But...that didn't really stop him from making this God Valley flashback as long as it turned out to be. And it didn't stop him from going to Elbaph in the first place, even though he was debating it. So...chances are this isn't something he's thinking too hard about, anyway. I have a feeling it will effect things. But...I think he's going to TRY and power through it. Given how he handled God Valley, he just doesn't seem like he's in a rush due to his advancing age and declining health.
Another factor is HOW Oda chooses to give us this flashback. Usually we get a flashback that starts from the perspective of someone who was around when it happened. But...that's not too likely, in this case, since it happened so long ago. So the Void Century "flashback" may come in a form of Robin telling us a myth. We may get the majority of the Void Century flashback as a "story", and then we only find out a few key "truths" later, probably on Laugh Tale. If so, the "myth" Robin tells us may be shorter, if only because it's a summary, and not the full story. We may never actually get the full story in that scenario. Just the parts that are important to the present day.
But, the other factor is that Laugh Tale reveal. If the Void Century flashback happens BEFORE we get to Laugh Tale, which I think is pretty likely, chances are Oda will want to hold at least one key detail back, so he can reveal it later on Laugh Tale. So, depending on what it is that he wants to reveal on Laugh Tale itself...we may be in for a shorter flashback than we expect. Just so Oda can save his big "surprise" for the last moment of the story. One last big twist that changes how we re-read this entire story. Oda would then have to have the flashback cut around this big reveal, so he'd have to cut away at key moments to preserve it. I think this might be the biggest factor in play. Oda loves a good surprise. So, we might get a shorter flashback, just so Oda doesn't give the whole game away before he's ready.
As for WHEN the Void Century flashback will happen?
I think the best chance will be on Lodestar, if that is indeed the next island we're going to after Elbaph. We know there's SOMETHING on Lodestar that revealed the importance of the Poneglyphs to Roger and his crew. It could be some special Poneglyph that prompts Robin to start giving us the highlights on a Void Century flashback. Lodestar would give us a chance to a fairly decent sized Void Century flashback, but not ruin the pacing by waiting until Laugh Tale.
Because Laugh Tale is really the only other option for when the flashback could happen, in my opinion. And I tend to agree, getting to Laugh Tale, only to take a year or more off to do another long flashback WOULD be frustrating. I would rather not have that happen. I think Oda SHOULD just save that one key detail to reveal on Laugh Tale, and reveal the majority of it beforehand. But...we'll see what Oda actually does. The God Valley flashback was kind of oddly timed. So, Oda doesn't always have the best "timing" for flashbacks, sometimes. You might say he even ENJOYS interrupting a big climatic moment or fight with a big flashback, like the face off with Saturn or the face off with Imu. So, Laugh Tale may still be the most likely choice, unfortunately.
So, if the next arc is Lodestar, that's probably going to be our best chance to get this flashback. Close enough to the end for it to be a exciting reveal. But, not so close to the end that it feels like a huge detour at the very end of our journey. Laugh Tale is probably still the most likely time, though, I hate to say it.
Okay, but I've rambled long enough. Let me FINALLY answer the actual question. How. Long. Will. It. BE?!
Well, that depends on how Oda chooses to handle it, of course. So, let's go over some options.
Option 1: We get a whole spin-off manga. 5 Volumes. 60 chapters. (Or MORE!)
[2 out of 10 chance of happening]
-I honestly think there's enough material in the Void Century and potentially the First World flashbacks to give us a entire spin-off manga covering the time periods. You could REALLY dive into this and give us some detailed material. There's so many characters you could introduce and so many subjects you could cover. So many questions you could answer! And I DO mean that "spin-off" part literally. I think the smartest thing Oda could do would be to actually get someone else to draw this Void Century manga while he's still doing the main One Piece story. We've gotten One Piece spin-off stories not drawn by Oda before, like the Ace novel adaptation and the Sanji/Food Wars specials. I say do that again, only with the Void Century! Would that kind of be a cop-out? Sorta. But, it would allow the main story to proceed, while also getting answers to questions we've wondered about for so long. Plus...I have a pet theory that the One Piece World literally looked a LOT different in the past. With all the possibilities that Devil Fruits and other "terraforming" having taken a effect on this world...it's possible that the humans didn't USED to look as "cartoony" as they do in the present day. Humans may have slowly begun to look more "cartoony" as a result of their dreams coming true, and exposure to Devil Fruits, among other possible factors. So, having a different artist do a Void Century flashback manga could actually be pretty fitting, meta-textually. The only problem is...I REALLY don't see Oda giving up the chance to tell this story himself. I think he SHOULD. There's arguably too much for him to cover on his own. But, I really don't think he will. So...this most likely won't happen. *Le SIGH*
Option 2: We get the whole Void Century flashback on Laugh Tale. Around 30 to 40 chapters.
[8 out of 10 chance of happening]
-If Oda chooses to do the entire Void Century flashback at one time on Laugh Tale...I could see it taking a while. I honestly don't know if I see it taking less time than God Valley. And given everything he has to do...yeah, I could see it being twice as long as God Valley, at it's longest. There's SO MUCH we have to get through. And Oda is NOT shy about going on and on with his tangents. So, yeah, I actually could see a scenario where we spend a whole year or more away from the Strawhats. And Oda DOES like doing flashbacks all at once. So, I think this structure is the most likely scenario...unfortunately.
Option 3: We get half the Void Century flashback on Lodestar, and then we get another half on Laugh Tale. Around 5 to 10 chapters on Lodestar. And another 10 to 20 chapters once we get to Laugh Tale to fill in the blanks.
[7 out of 10 chance of happening]
-I feel like this would allow us to keep things a little shorter. Split things up. Kind of like what Oda did with God Valley. Splitting it between Kuma's flashback on Egghead and Loki's flashback on Elbaph. Mind you that STILL led to a VERY long flashback. But, not as long as doing it all at the same time, I think.
Option 4: We only get a shorter "mythologized" version of the flashback from Robin on Lodestar or Laugh Tale. We never actually get a full flashback. 2 or 3 chapters of Robin talking and telling us everything she's learned from the Poneglyphs she's read.
[1 out of 10 chance of happening]
-I could see us never actually getting a full-on flashback. Instead, we could just hear what Robin's learned. Having the Void Century and First World retain their sense of mystery by never actually being delved into in great detail. And that's that. I think that's VERY unlikely. But, if Oda gets to be in a BIG rush, due to his health or something, I could see it being just barely possible. And Oda does sometimes just...skip things. Like the Payback War, or the Rocky Port Incident. I think Oda almost HAS to cover the Void Century in some detail outside of a SBS. But, it is possible, though unlikely, that it won't be as much of a focus as we all might think. It's also possible that we really don't NEED a long flashback, because Luffy's already lived through a parallel version of most of it. But, I do think this is pretty unlikely. Oda's been keeping this a secret for so long, I think he's got a story cooked up and ready to go. So, I honestly doubt it will actually be this short.
Option 5: We get the Void Century Flashback and the First World flashback during Laugh Tale, but it's used to parallel Luffy's current journey. 60 chapters, but it's alternating between the present day and the past. Effectively around 30 chapters.
[3 out of 10 chance of happening]
-This is the option that I find the most interesting and that I like the best. But, it's also something Oda's never done before. Instead of doing a flashback for several chapters straight, instead we basically have one chapter of flashback and one chapter in the present day, and alter between the two for most of the arc. We see Nika and Joyboy try to do something...and then we see how Luffy chooses to handle that same situation. Comparing and contrasting how each of these characters are both similar and different. We see Joyboy tying a Haki Knot in the past...then we see Luffy untying it in the present. I think that could be a VERY interesting way to structure the final arc. That way Oda doesn't have to reveal something that will spoil the story. But, we can also keep the story in the present day moving at the same time. One chapter in the past. Followed by one chapter in the present. Slowly revealing the steps for what happened and how Luffy will deal with it, as we see it unfold. Maybe since all 3 have the Nika Fruit, they could even interact with each other on this very special island. Completely shattering the fourth wall by having Nika and Joyboy literally handing Luffy different items that he’ll need from the black panel borders over to the white panel borders. I don't think it's very likely. Oda's never used a structure like this. But, if I was the one writing the manga, this might be the option I'd choose. We don't get one uninterrupted flashback. We get to see the how 3 generations of people chose to handle this similar situation. And we see why Luffy succeeds where Nika and Joyboy failed basically in real time. That way we, the audience, discover these things at basically the same time the Strawhats do. Not before or afterward.
So, there you go! Somewhere between 2 and 60 chapters! Around 30 to 40 chapters probably being the most likely, on average? So helpful, I know! But, I know some of you skipped to the end of this, so there you go! Now, you don't have to try to read through my 20 tons of text!
I'm probably wrong about all this. But...it's hard to predict Oda at the best of times. And Oda's probably been planning this flashback since almost before the manga was actually made. So...I'm definitely looking forward to it.
