If you allow students to schedule make-up lessons, you can set the expiration for their make-up credit along with a reminder email, again with a custom set advanced notice, so they don’t forget to use the credit.
To get to these settings, click on your name in the top right and click on Settings.
Then click on the Calendar tab.
Scroll down to the Policy settings, and look for the Make-Up Policy section and enter in your desired policy.
Finally, click the green Save Calendar Settings button at the bottom.