I never try and stay in them longer than I have to. Some people like to tough it out so to speak, but I'm almost immediately looking for where I'll be getting my next diaper change. It can be in a bathroom, office, etc. if I must I have been taught how to change in my car, or obviously at home. Never been in a situation to use a changing table or anything of the sort either. I'd normally say I'll be in a poopy diaper for anywhere from 5-35 minutes if I choose to drive home.
I seldom mess while I'm asleep. I'm usually triggered by meals so that's unlikely but if I have late dinner or eat before bed it has happened. When I have messed it hasn't had any issues with wetting leading to leaking. It's just a full diaper really. I don't think having a mess in the seat of your diaper truly leads to a leak if you wet. I guess I'm trying to say I don't think that mess in the seat would really have any affect on any pee going into the diaper. Since the poop isn't getting absorbed like the pee, it sort of just sits in the diaper. So other than getting I guess a wetter mess in your diaper, I don't think it really changes much. At least in my experiences that is.