Browser sessions for both admin and portal users persist for four hours.
In other words: all site users, admin or otherwise, will be automatically logged out after 4 hours of inactivity after entering their password. Users can always end their session early by clicking the 'logout' icon.
If a user closes their browser tab without logging out of the site, then (depending on their browser's settings) they may stay logged into the site if they reopen that tab and/or navigate back to the site in a new tab. The automatic timeout is set to 4 hours so that users do not have to log back into the site every time they step away from their computer or switch tasks, but also so that a computer left unattended does not remain logged into the site indefinitely.
The session timeout length cannot be changed on a per-role or per-user basis.