I'm hoping the campaign is about 1.5 the length of KH2's, or at least double DDD. The series has a lot it needs to wrap up, as many have already stated, as well as whatever plot twists Nomura throws in. I don't think length is really that much of a problem, it's how the story is handle. KH2 had a long campaign, but it felt even longer because majority of it, aka the Disney worlds, weren't even that connected to the actual plot.
In the case of KH3, however, I think that Nomura and co. have learned their lesson in that regard, since I never once felt the worlds in BbS and DDD didn't serve a purpose.
Now, what I would like the length to be gameplay-wise would be something similar to KH2FM's or BbSFM's. There was so much to do at the end of both of those games. Plus, I strongly believe Mirage Arena will make a return, so I doubt the postgame would be as short as, for example, Days and DDD. Although, it would be nice if they could throw in something like Re:coded's ability to refight bosses and such.
45+ hours is far too long for a story.
Atlus would like to have a word with you.