I have used the refresh token several times in just a short period for testing purposes, but I wonder whether Google refresh tokens ever expire? Can I use the same refresh token to get another access token again and again for a long period (a week or even months)?
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.