@Long:
We could argue about jack forever but
I want to hear some solid predictions here.
How many chapters until jack is defeated.
I'm gonna say around 15
Hmm, it depends on what happens with Jack's plan to kill Zunisha, and where Luffy/the Alliance/whoever ends up engaging him for a fight. If Luffy/whoever manages to stop him before he gets too far in his efforts, the fight starts quickly, and it doesn't get interrupted, I'd say probably fewer than 15. (Maybe 12, like your second guess?) A straight fight probably wouldn't take too long. But it's possible that it'll take a while for the Alliance to even get to Jack, since he's on ships attacking from a distance (most likely) and they're all the way up on top of Zunisha, so that could delay things.
One option to consider is that the fight will be interrupted somehow, like for example if Jack tries to capture people instead of kill them. Like how, on Punk Hazard, it didn't take Luffy that long to engage Caesar the first time, but then he got knocked out and captured and so the final fight wasn't until much later. Still, Jack's so batshit insane that this one's probably less likely, since he'll more than likely try to just kill everyone except the four samurai.
In considering all those possibilities, I'm guessing Oda-sensei most likely will wrap up this arc at least somewhat soon, so that Luffy can leave to go get Sanji, so I'm leaning towards one of the relatively shorter options.
–- Update From New Post Merge ---
Also, anybody wanna take a stab at guessing who's gonna end up in Luffy's Whole Cake Island group? Right now, Luffy's intentions are very clear: he still wants to go alone with Pekoms. But do you guys think that'll actually happen? Will at least one Mink go along, or a samurai? A Straw Hat, or a Heart? Multiple of the above? I have to say, after reading through so many chapters in the Paramount War saga where the whole crew is missing, and then Dressrosa where half the crew was missing again, I'm not sold yet on the idea of putting them all (except Sanji) on a bus while Luffy goes off by himself once more. I kind of want at least some of them (maybe the ones who were gone for Dressrosa, at least) to insist on coming with him. I think it'll also depend on what happens with Jack and Zunisha, though: whether the Minks lose their home, or Luffy is injured in the fight and can't go by himself, or the attack convinces them to stick together, or what.