The italki policy is that all changes to classes must happen a minimum of 24 hours before the scheduled time. This is to protect the teacher, who may have lost the chance to train someone else in that time, and may have cancelled other personal plans to be available for the lesson. Of course, this courtesy goes in both directions: the student would not like the teacher canceling just a few hours beforehand, either. Therefore, if you cancel less than 24 hours ahead, the teacher will still be paid for the lesson.