I think attendees at the event managed to spend an hour in the Toy Box world (though that includes over 22 minutes of cutscenes) which was just under the average length of a world in KH2 and BbS. Since that was only 30% of it, and there's still more bosses and areas to explore, I imagine the full length of the story campaign in that world will be 2-2 and a half hours, with 100% completion being a lot more.
Finding treasures will definitely be a task. Hopefully it's improved over Dream Drop Distance's treasure list. I remember Country of Musketeers on Side:Sora had a chest in the journal with the other chests in the Mont Saint-Michel, however the chest was actually over in the Opera House.
There's also the little secrets and nooks and crannys to explore. We know about the Rock 'Em, Sock 'Em robots that house a treasure chest, but there's certainly a lot more to explore and find. There was a closed off store on the third floor of the emporium, and you couldn't go far inside the roof.
Like you said, there's also the vents which have puzzles inside them too (the ones where you have to hit a bunch of switches in succession or at once, idk no one really did that well).
It's absolutely going to be time consuming. And the other worlds don't look any easier aha. Olympus has chests hanging around in odd spots, plus you have different parts of that world to explore which means lots of traversal and opportunities to have things hidden away.