In my contract, I state that I require 24 hours notice for a cancellation.
However, in practical terms, I enforce this rarely. When a client calls me in the morning because he/she is sick, of course I do not charge for the session. I also sometimes get calls with unexpected work-related issues which makes it necessary to cancel.
I have enforced this policy at times when I had repeat-offenders who always had one reason or another (all seemingly valid in their own right) for the late cancellation. I had let that go for a while but eventually announced that I would begin to charge for missed sessions as per contract.
I am happy to say, though, that I usually do not have a problem with late cancellations.