My Ts.practice's policy is a charge of you don't cancel 48 hours in advance. Each T came do want they want, however and mine has never charged for my late cancellations (very few). He said he doesn't charge for illness, sick kids or if you're stuck in traffic- basically anything reasonable.

I saw a t who told me a story about a client who came to his appointment really sick, so she asked him to leave and reschedule. He said ok but asked for his money back and she said no- that it was still her time and she should be paid for it. I thought that was really unreasonable on her part.