Whatever suits you. In terms of vfm I agree with the poster who said 15mins bookings often trim back and (in my experience) so do 1hr bookings; whereas half hour bookings have the highest rate of running over the specified time. Generalising of course but there's probably good reasons for it when you think about it. In terms of interest, I can think of plenty where 5 minutes is enough to realise they were too boring to spend longer with, and some where all afternoon was only enough in terms of needing to sleep or rest the balls at some point. Punter needs and prossie deeds.