For a first punt with any girl, I do 30 minutes since I'm not wasting an hours worth of money for someone who I have to walk out on within 10 minutes.
What's more important than time is how many times you can cum per half hour or even an hour. Unless a profile explicitly says "Unlimited cums" then assume that max for half an hour is 1 and for one hour it's 1 or 2.
This is also my game plan.
Prices are now starting to creep up to ridiculous levels.
30 mins is enough for a vanilla session of kissing, fondling, oral and sex before filling the bag. I also always book 30 mins with a new girl.
I do book more for a regular or where other stuff is on offer, such as BDSM etc.
Like people have said, some WGs have a bad attitude on shorter punts, a bad attitude on 30 mins belies a bad attitude in general.
It also helps screen out nonsense like "come once" policies - we are supposed to be paying for time.
Takes me a while to come (despite being quite young), so it's not a problem for me in 30 mins - but YMMV.
My advice is start with 30 mins and work upwards if you feel you need to.