Yeah, I don't do exits, either.
I don't charge for a specific number of hours. My deal is I show up at the bride's place two hours before leaving for the ceremony and I stay until "the reception winds down." After the cake and bouquet, I ask the couple if there's any last group or other shots they would like. Usually by this time in the evening, I've taken all the dance shots and people talking/visiting shots I possibly can. So I assess the room; if it's just going to be more of the same I leave. If people are getting ready to dance on tables, I'll hang around a little longer. 



