Bearer tokens expire after a relatively short amount of time (currently 30 minutes). After a bearer token expires, you will need to use a refresh token to get a new one (refresh tokens can be used to get an unlimited number of bearer tokens).
If you’re exchanging an authorization code for a bearer token, and you did not request duration=permanent, then you will not receive a refresh token in this response. Your bearer token will expire, after which time access to the user’s account will no longer be available (unless you authorize them again).
If you’re exchanging a refresh token for a bearer token, you may or may not receive a refresh token in this response. If you don’t receive a new refresh token, then the refresh token you just received is still valid. If you do receive a new refresh token, then your old refresh token is no longer valid and has been replaced with the new one you just received.
Refresh tokens do not expire from disuse, but they do get rotated periodically after being used in the manner described in the preceding paragraph.