He has never mentioned his actual policy or whether he even has one.
If I know beforehand that I can't make it, I usually cancel either the session before, which is 2 days, or the week before, so 7 days. My T usually informs me at least a week before if he knows he'll be gone, so I try to do the same.
Once I was sick and only knew that morning that I couldn't make it to the session in the afternoon. We arranged a phone call instead and there was no talk about 'you have to tell me earlier' or something like that.