In case the player disconnect abruptly (that is, without sending system/disconnect request), the session will then stop a short time after.
But if the player is currently in game, this timeout logic should instead be tied to the battle in progress, not to the connection. This would avoid a player crashing then having major trouble restarting the PC and game, so they can always rejoin the battle.
If there is still no connection when the battle ends, the usual timeout should be resumed.