Generally-speaking: a lot of fun.
Usually, I only undertake long shoots with people that I know will be fun and energetic and where we’ve (usually) roughed out a number of concepts and/or locations to play with.
I find I’m more relaxed and comfortable during longer shoots — the extra time gives us the freedom to be creative without having to worry that we’re wasting limited time (and possibly money) on something that didn’t quite work.
To be fair, when I say “8 hour shoot" there’s usually a chunk of that time that’s taken by travel time to and from locations and usually at least one meal. (If Bunny comes to visit, it’s almost invariably burritos at Cosmic Cantina.)