The Google Auth server issued Refresh tokens never expire — that’s the whole point of the refresh tokens.
The refresh token will expire (or I should say become unauthorized) when the user revokes access to your application.
Refer this doc it clearly states the function of refresh tokens.
Instead of issuing a long lasting token (typically good for a year or unlimited lifetime),
the server can issues a short-lived access token and a long lived refresh token.
So in short you can use refresh tokens again and again until the user who authorized the access revokes access to your application.