I'm pretty well situated along a major artery so, like Jack Bauer in the 24 TV show, I can be just about anywhere in 30 minutes, if I've timed the appointment to avoid rush hour traffic. Also, my other hobby is going to estate sales, so I know the entire metro pretty well by now. And by metro, I mean the 494/694 loop, and a few miles beyond.
My ATF used to have an incall that was way south, about a 45 minute drive, but man, she was so worth it. On the other hand, when I was experimenting with variety in providers, I once drove way the hell out into the country to see a girl. Took me an hour, she was on the creepy side (fetish gear, wouldn't take her boots off because she hated her feet), and the place was so obscure and out of the way, I felt a little nervous. I wouldn't do that again.
I think your number is about right. More than a half hour drive would be a bit out of my comfort zone seeing a new provider, as it suggests you might be going out of the metro area. Number of reviews would be a factor, though. Upwards of 20 glowing reviews, buzz on the boards, I might make a longer trip.