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The Refresh Token does not have an expiration date; however it can be made invalid if another Refresh Token is generated.
When you create your first Refresh Token it will remain active and valid as long as it is the only Refresh Token. If for some reason you generate another Access/Refresh Token using the oAuth flow that will invalidate the first Refresh Token.
Refresh tokens expire after 180 days, if not used. See docs.
Once a refresh token is used, to extend the life of an access token for example, it becomes invalid.
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