I cancelled on occasions when I was sick or some other unpredicted issue came up - not terribly often. Official policy was 24 hours notice, but I was never charged because it was such a rare thing for me to do. My husband was notorious for forgetting his appointments (in his defense, we are about to have him evaluated for early-onset dementia which had probably started in its early stages back then). In my husband's case, T found him a regular day and time slot to help him remember his appointments better, but he would have charged him if it happened too frequently.