When does my medical form expire?

Your medical form can be good for up to five years, it depends on your age. If you as a person compete in the "historic class" (like, over 70), the length of time will be a lot shorter, like a year.

The exact expiration date is calculated from the date the exam was performed. This does mean that as soon as the exam is done, you may as well send it in immediately, as waiting until just before an event does not buy you any additional time of validity. The date of the credential is always tied to the date of the actual exam, not when it was processed by NASA.