It works both ways; if she's not there, you don't pay her. The T does not "pay" you if you are not there and you do not pay her if s/he's not there. Presumably both parties do warn the other when they are going on vacation/not going to be able to attend the scheduled meeting.

My T only had the 24 hour policy, and only stated it after 7+ years when I was having problems with a serious illness and last minute cancellations. It's not like, if they have a set time put aside for you that they can get someone else in there, everyone in their practice has a set time, the same schedule. So, if you take a vacation, it's a forced vacation for the T too, but only for that hour. I would not like to suddenly have random time free in my work day; not enough time to do anything substantive, on the fly. "Oh, by the way, don't do anything between 2:00 and 2:50 this afternoon, your 3:00 and 4:00 client will be in at their usual times. . ."