This read-only value reports how long, in seconds, the current 802.1X session has been active since it was authenticated. An administrator checks this to see how long a device has held its authenticated state, useful for spotting unusually short-lived sessions that might indicate flapping connectivity or repeated re-authentication. For example, a session showing only a few seconds of duration right before disappearing from the table would point to a client that's authenticating and immediately disconnecting.