It depends on the therapist. Mine is like growlycat's with 24 hour notice, and that's been the case for every therapist I've seen; however, they don't get militant about it unless someone is just blowing off the session--a no show. You at least called as soon as you noticed your mistake. Mine was flexible when I was sick and didn't know how I would feel, but we communicated on that and she offered to hold my time even if I couldn't make it. I guess it all comes down to how it's handled/communicated and what the therapist has said is their policy. I wouldn't worry about it if he hasn't said anything about paying.