I always go for an hour, which gives time to get the quick BJ you'd have in a 15m quickie, then recover and spend most of the rest of the time fucking.
There are some girls who I've considered the 15m quickie with, but only if it's a cheap option - and I mean like £20ish. Trouble is, those girls are a similar drive away for me as the ones I see for an hour so it isn't worth it. If I lived nearer to them (or if there were local girls who did it at that rate), I'd do the 15m option all the time.
Although I have sometimes wondered about going for the 15m quickie on the way to a 30m booking with someone else, that way I'd get what I want, have the opportunity to try 2 different girls and potentially save money. It's just planning it and getting the timing right with two different girls that would be a pain.