Some girls only offer longer bookings. I haven't had a booking of 30 minutes cancelled from those who do offer it.
My preference depends on my mood and the girl's feedback. If it's someone i haven't seen, 30 minutes is a good trial option. Or i might not have time for a whole hour. And some girls basically stop after one pop so an hour is wasted.
If i like the girl, an hour is the norm for me. Actually, 45 minutes is ideal as it combines the intensity of a 30 minute session without the dead time of an hour. Although not an option on aw, mist girls I've asked will agree to a 45 minute booking. Pricing varies, sometimes it's almost the same as an hour in which case I just ask for that.