Short answer
How much notice is required, what happens without it, and what counts as an exception — all three in one paragraph, announced in the first lesson.
Why it must be written down
A spoken rule is renegotiated at every cancellation. A written rule is discussed once and afterwards only recalled.
Being fair matters more than being strict. A rule that looks one-sided does not get applied, and a rule that is not applied is ignored.
How much notice
The common range is 12 to 24 hours. Less than that leaves no chance to fill the slot; much more is unrealistic for the client.
For group lessons the window is longer: one person cancelling affects the class itself.
Write the exception too
Illness, emergencies and force majeure are exceptions. If the exception is not written down, every cancellation tries to become one.
State how many times it applies — “once per term”, for instance. An unlimited exception voids the rule.
Sample wording
“Lessons cancelled with at least 24 hours' notice are rescheduled free of charge. Lessons cancelled later, or not cancelled at all, count as held. In case of illness or emergency this rule is waived once per term.”
Read it in the first lesson and share it in writing. A rule sent later does not cover cancellations before that day.