I think it really depends on the girl - some do better on 30 minute meets.
Personally I think that only plays a small role. If the girl is noticeably worse during a specific length of time, especially longer bookings (but whatever length really) then she's probably not all that great anyway.
I can't think of any WG who I've seen multiple times for various durations, who's been noticeably different. The only noticeable difference being just the time obviously. Again, the key thing is that they're all good WGs.
I do think you have a point when it comes to some girls though, particularly those who have mixed/not glowing reviews though. Often shorter bookings can be the best way to get the "best" out of them but they're still never great in general terms. I'd normally just advise choosing a better girl to begin with.
As others have also said, a good WG will often not have issue with running over a bit, especially if you seem close to cumming. Sometimes they might be very busy afterwards but good ones should still leave adequate time between punters so it shouldn't be too much of an issue. One shouldn't ever expect extra time though nor should you take the piss by deliberately booking less than you think you'll need.
Generally if it's your first time seeing a girl it probably is best to go for 15 or 30 mins unless the price disparity between that and the hour is very small.
I definitely agree with that as it's what I do (30 mins), unless as you say, the price difference for an hour is very small (or it's fairly small and I just have her as the 2nd punt of the day). However it obviously depends on whether the punter can fit in everything he wants and can cum within that time.