Ask HN: As a freelancer, what are the terms of your service contracts?

I have got a maintenance/service contract with a company. I've basically agreed to work up to 15h/month with a 24h response time on any maintenance task (bug fixing, server issues, etc.). Whether I work 15h or less during the month, the amount they pay monthly is fixed.

What I'm wondering is how about months when there's almost no work, say there was only 1-2 hours of maintenance to do. The client might be a bit frustrated because they pay me but there's not much to do.

Do you have any clause in your contract to handle this? I was thinking, for example, if there's one hour left or more, it goes to the next month, so there will be 16h next month. Or maybe I do some extra development work at no extra charge. No sure what is the best practice for this, and what would be fair.

Any suggestion?