Connectivity problems and crashes are an unfortunate and unavoidable reality of Second Life, and I try to be as understanding and respectful of my clients' time as I possibly can.
In the event that I crash or disconnect in the middle of an appointment, I will make every effort to return as quickly as circumstances permit. Once I'm able to return, I'll resume the appointment if it's practical to do so, or reschedule if it's not. I will never charge a client for time during which I am offline or otherwise unavailable.
In the event that a client crashes or disconnects in the middle of an appointment, I will wait for 15 minutes or until the scheduled end of the appointment, whichever comes first. Should the client return during that time, I'll resume the session; otherwise, the client should contact me to reschedule the remainder of the appointment, if any.