It's quite simple. You have strict time slots for each act. Within that time slot they set up, play and get off. End of story. If their slot is 30 minutes and they set up in 20, they get 10 minutes to play and get off.

I help to run an annual one day "back garden" festival each year. The rules are simple - 15 minutes or 3 songs -then you're off. We get through the usual 20 acts or so and finish dead on time.