I would wait 15 mins max, I've learned the hard way, dropping punting money faffing around waiting.
I text on the way, text on arrival then call 5 mins past booking time. Wait 5 mins then text/call again.
I'd rather have radio silence than give me 2 mins I'll call you back, and 10mins later, just 5 mins more I'll call you, 25mins later I'm ready, like a lost donkey walking around in a unknown area for 25mins no freaking way!
I make my way back exactly 15mins past booking time, unless of course she has raving review's about her and no one has said she's stood them up before, I'd be prepared to wait for about 25mins max