Currently AFCS uses some generic authentication services provided by the Adobe infrastructure that we don't control (I mean, we as the AFCS team). But I think the session terminates after a certain period of inactivity (just don't ask me how long)
If you are doing requests that are spread in time I would just say, login every time.
If you are doing a lot of requests to the server (why ?) and you really want to cache the session I would suggest you "manage" authentication errors to know when to authenticate again, or do something like login again if a session is older than one hour or so.