Lots of good points in here, and good suggestions on how to handle the situation discreetly and delicately.
I think it's perfectly fine for a lady to ask the client if they would like to extend the booking or offer extra time, but NOT if time is up right at that very second, and the encounter is being cut-off right in the midst of it. PLEASE don't be like: "Ok, you've been here 30 minutes and 20 seconds-- you're over the time! Get dressed and GET OUT!" That's going to cause nothing but hassle, annoyed punters and bad reviews, and possibly worse trouble.
If certain escorts feel the need to be that strict with their time to the point where they would be clearly displaying clocks and/or timers in the punters face (a SERIOUS turn-off for me and I imagine many guys), than at the very least, a 'five-minute warning' would be greatly appreciated, and it's possibly at this that the invitation to extend the booking can be mentioned. And if not, the message should be clear enough for most guys that this means 5 minutes to 'seal the deal' and 'lock it up'.
But please, do NOT get into a discussion about money right then and there for sure, serious mood breaker! If a lady is going to be this strict with her time, than a subtle mention that time is ***almost*** up will probably resolve the issue before there are the chance for any problems to arise!