24 hours notice, but one free "pass" a year. I also think that an emergency-- I'm hardly ever sick and I think I've only canceled once in all these years because of that-- my T would not charge me for it. My sense is that a harsher cancellation policy gives more room for leniency when needed, like for people who have a hard time making appointments consistently. It's easier to give someone a pass from the policy rather than to start ratcheting up the consequences from a lenient one. That's what I did many years ago when I was teaching college students, like never ever will you get an extension on a paper or a rescheduled exam, unless you have a dean's excuse (and deans, in my experience, will hand these out just to get students out of their offices, but students seem loath to ask).