If you request offline_access, the user will see "access your data at any time" as a requested permission. If you do this, the access token you are given does n

If you request offline_access, the user will see "access your data at any time" as a requested permission. If you do this, the access token you are given does not expire unless the user does some action to expire it (such as updating her password). If you don't do this, the access token has an expiration date, and the user has to refresh the session (transparently happens with the JS SDK, I believe) in order for you to have continued access.