This is one of the reasons I don't session much anymore. I've had a few dommes end a two-hour session after an hour and 45 minutes. When I asked, they said they needed to get ready for the next client. I didn't totally believe it, but I didn't feel like making it an issue.
At this price-point (NYC rates), dommes should be giving the full time. I understand that pre-session conversation starts the clock, but ending early is taking advantage of the client.