This happens occasionally to me, and I take a view on each individual client, for instance i have a reg, see him every week for 30 minutes, he is in and out in 15 minutes (and thats including showers), last week he must have been feeling really horny, with all these ideas what he wanted to do (when really I know he is a 2 pump and cum guy), so he booked 1 hour..
..sure enough he was done in his normal record time and there was no chance of a round 2, so I refunded the extra half hour he had paid me for. I felt I make enough money very easily from this client, so dont want to make him feel hard done by, he left really happy that I had given him the money back, and I know he will be back. On the other hand, if it was a new client that this happened to I would try and pad the booking out....but no refund would be given.