Oauth operates on the issue of token that are updated after you initially authorize access for as long or as little time as the server operator decides. It has nothing to do with Thunderbird. I have found with my Google accounts that; * Changing my ISP triggered a reauthorization. * Changing my device (new computer) triggered a reauthentication. * And through anecdotal sources using a VPN can also be a trigger. All things that Google might consider a "change" to your access which might indicate a hacking attempt. I do not know what your individual circumstances are, but you will have to reauthenticate as often as your provider requires it.