It can work both ways...
If I find a gem that I like, by 2nd or 3rd, quite often I may have negotiated a good, lower rate for a 2hr appt.
These future visits man easily be "cancelled" by the girl, at the last minute sometimes, cos she may have a higher paying punter, although the reason given is quite often different (surprise visit from a friend/family member, time of the month, etc.
If you are happy with this, which I usually am, provided appt is not cancelled that often, then it can be a good arrangement.
Agree with all the others that if service begins to fail, it's time to move on. I once had a regular who was not too happy with a cum twice request on my 3rd visit (in a f*****g 2hr appt!!). When reminded although she did provide it, that was my last visit to her and I did let her know that she would normally provide this to ANY punter in a 1hr appt, let alone a 2hr one.