Depends on how the appointment goes.
I have had appointments ranging in length from 30 minutes to a couple of 12 hour overnights.
In some of the shorter appointments that I had the time absoutely dragged just because of the fact that I did not click with the escort concerned.
Personally I would go with the 30 minutes option as if it turns out to be a disappointment at least you have not spent as much money and you can always choose a longer appointment if you decide on a repeat visit.