I guess you deal with smart guys!
it's understandable and thank you for sharing. Unfortunately Viagra delays the orgasm, besides you don't know if the guy has someone else scheduled for later or he just came. Busy days do happen to escorts.
In your place I would offer a generous tip to make him save it for you.
I'm not a "minute man", I always say that if I was straight I would be successful because even when I was young I would take my time to cum. Maybe that's why I understand why for some escorts ejaculating with a client might be difficult.