Depends on how much of a completionist/perfectionist you are. I'll give you a base 20-30 solid hours and don't try to rush it, maybe less if you just do nothing but story missions and ignore all the sidequests, hunting, poker, bounty hunting, etc, etc, etc... Doing all of it though, or even a good chunk of it? A looot longer. Lol
On top of the sasquatches, you get "special" horses.
No it doesn't. The Xbox One version is a emulation. You are literally playing the 360 version on Xbox One when you run it. It doesn't take advantages of the new hardware aside from a more consistent 30FPS. But it's not like Rockstar or Microsoft did anything to get it to be more consistent.
Who knows if Sony figures out how to get PS3 games on PS4 but I highly doubt they will. Both consoles are completely different.