Yes, I strive to be quite flexible. What I don't like is when people call me a few minutes or a few hours before their lesson to tell me they can't make it. Why? It doesn't give me enough time to fill that particular time spot. Then, I am stuck with an hour being unproductive.
I prefer at least a 24 hour notice. With that amount of notice, I can plug in another student who needs a make-up or I can schedule an appointment, or well, you get the idea, right?