Imo, I don't think it can be that long now. The Rocks flashback honestly has to with far more than just Kaido (seems it may even be related to Shanks and Blackbeard in someway).
I think what we'll get is a very abridged version of it, that more highlights Kaido, Linlin and Whitebeards relationship. But I don't think it'll last more than a chapter unless it's done after he loses. That would ruin the pacing of the fight.
A big example of this is just Yamato's flashback or Zoro's. They are quick and placed in the middle of a fight chapter. Kaido's is far more important obviously, but it may just focus on Kaido, not Rocks (only tease that).
I think what we'll get is a very abridged version of it, that more highlights Kaido, Linlin and Whitebeards relationship. But I don't think it'll last more than a chapter unless it's done after he loses. That would ruin the pacing of the fight.
A big example of this is just Yamato's flashback or Zoro's. They are quick and placed in the middle of a fight chapter. Kaido's is far more important obviously, but it may just focus on Kaido, not Rocks (only tease that).