Apply this logic to the Gorosei, and then tell me how short Egghead was.
Well, to be fair, Saturn didn't even show up on Egghead until around half of it was done.
Egghead started with Chapter 1058. Saturn's name was first revealed in Chapter 1073. Saturn first got directly involved with the fight and revealed his "Yokai" form in Chapter 1094. The other Gorosei arrived in Chapter 1109. Egghead ended with Chapter 1125.
Egghead was 68 chapters long. Personally, I'd say Saturn only really started to be involved in the story once he started fighting in 1094. 36 chapters in. A little over halfway through the arc.
Which means, if the Gods Knights are already getting involved in the story, this may be around the halfway point of the arc. If, as you say, we apply the same logic as Saturn arriving on Egghead. It might not be structured similarly, but it could be, we'll see.
And unlike Egghead, we possibly won't be cutting away to see what's going on the outside world until after it's over. Especially if the whole "time dilation" theory is true and weeks are passing in the outside world while the Strawhats are on Elbaph. It's possible we won't get any scenes from outside Elbaph until after it's all over, and we get to see what the Strawhats missed in their time awy. If that theory turns out to be true, which is admittedly a big assumption. But, chances are, we'll probably remain focused on the events in Elbaph while we're still in this arc. Which means Oda won't have to split his focus as much.
If we want to go with another comparison, Zou went from Chapter 802 to 824, for a total of 23 chapters. Jack, the main antagonist of that arc, didn't arrive on Zou in the present day until Chapter 819, and didn't really start doing anything until chapter 821. Now that is a bit of a extreme example. And I doubt we'll only get 3 to 5 chapters of the Gods Knights actually doing stuff. But, it does show that Oda does sometimes wait until almost the end of the arc for his villains to do stuff in the arc they're in.
Let's look at Dressrossa, this time, instead. That arc went from Chapters 700 to 801, for a total of 102 chapters. Personally, I'd say the Strawhats fight against Doflamingo and his crew "directly" really started with Doflamingo putting bounties on their heads in Chapter 746, 46 chapters into the arc. Maybe you want to start with Usopp knocking out Sugar, which is Chapter 742. But either way, it's approximately halfway through the arc.
Let's look at Whole Cake. It went from Chapter 825 to 902, for a total of 78 chapters. The Wedding started in Chapter 856, 31 chapters into the arc. Again, close to halfway through the arc. A little less this time.
Let's look at Wano. It went from Chapter 909 to 1057, for a total of 149 Chapters. The Raid on Onigashima started in Chapter 974, 63 chapters into the arc. Again...about halfway through the arc.
So, yeah, I'd still say it's pretty likely that this could turn out to be a shorter arc. Oda has been fairly consistently waiting until around the mid-point of a arc for the Strawhats to start facing off against the villains of the arc.
If the Gods Knights are already on the move, and if the Gods Knights are actually the main antagonists of this arc, chances are we could already be in the middle of the current arc. Maybe somewhere around a third and halfway. Oda does seem to have a pretty consistent formula for his arcs. I have no reason to think Oda will change this, just because he's introducing the main villains of the arc "early" into this particular arc.
I doubt we'll get another 50 chapters of JUST the Strawhats fighting the Gods Knights. If more factions show up, like Blackbeard and Shanks, then maybe it'll go on that long. But, if it's just the Gods Knights? Maybe another 20 chapters. Chances are less. That's my best guess.