I suppose it's all down to the lady's discretion. How busy is she that day, how long you've booked etc.
Technically it should start from the time you booked. This sort of topic has come up here before, as have threads on client cleansiness and hygiene. So, imo it seems logical that you'd be allowed a quick shower beforehand, otherwise the dirtier clients out there probably won't be coming out of the bathroom squeeky knowing they're on the clock...