Sorry guys I slightly disagree here. I don't believe in clockwatching to the extreme, if the hour is up and he is just about 'there' well in the words of mastermind - we have started, so we'll finish. But yes the sex finishes generally in the alloted time, but if I don't have particular plans or am expecting someone else I don't mind chatting while getting dressed, showering, etc. Obviously I won't let it drag on too much. Obviously if the client wants it to finish bang on time or sooner that is fine by me, he is paying after all.
If the extra time suites me I would never accept the offer of extra money, if he insists on giving it I'd maybe suggest he invest it in a little gift for me - chcos or flowers maybe for the next time.
However if he asks me to extendour booking at the end of it and I have the time I will ask for extra payment, there and then, before we continue.