I've got a booking this evening with a girl I've seen before that I made two days ago and i was confirmed.
Sent a text this morning asking her to confirm, no reply and sent another mid afternoon as well as one on AW again asking her to confirm with no response. 
This proper pisses me off, bookings made in advanced, confirmed, and then no response on the day.

A dick wont suck itself so you should be given at least a few hours notice so you can make other arrangements.
Had it happen to me more than once so if they dont let me know in plenty of time I just black list them for bad behavior, life is too short fella and theres plenty more girls willing to take your money.
Dont get me wrong, I don't mind if they let me know in advance that they need to cancel or reschedule because shit does happen. Some of the really good ones will even cancel their bookings for the whole day if they wake up in the morning (or afternoon some of em) and find a spot on their chin.