how many hours of gameplay? And I heard he's going to give the final, right?
Depends entirely on how many girls you're with ... and how many paths you play. That said, I've never measured, but there's up to 11 different paths that can be played.
Two of them are unique to themselves (Val and Zenda). Val's can cross into the other girls, but if you get on any of their paths it'll end cause Val to let you go (possibly after you give her a baby). Zenda pretty much cuts you off from the other LIs if you go with her.
The other four can be mixed and matched to some extent,
and you can ditch (or be dumped by) them all and go solo. But the main thing is,
other than Sadie, if you're not on their path, you pretty much loose most of their story content. So basically, the more girls you get with, whether in one playthrough or multiple playthroughs, the more content you get, and the more hours of gameplay.
Best guess (with absolutely no attempt to keep track) is between 4-10, depending.
And yes, the upcoming release is the final chapter.