Server_Session_Timeout 

Explanation:
Specifies the number of minutes of inactivity after which the server automatically terminates network and mobile connections. The minimum recommended setting is 30-45 minutes. A lower setting may negatively impact server performance. The ideal setting depends on factors such as server load and the number of  concurrent users on the server.

UI equivalent:  None.

Default:
Default:  No default entry, but in the absence of the setting, Domino terminates a session connection after 240 minutes of inactivity (four hours).

Recommendation:
Set Server_Session_Timeout to 30 or 45

Restarts:
Changes to this setting will be automatically detected.

Related Settings:
MailLeaveSessionsOpen is a router specific setting and does not interfere with the DB server specific setting Server_Session_Timeout, or visa versa.

References:
Technote 1293213
How does the notes.ini file parameter 'server_session_timeout' affect server performance
http://www.ibm.com/support/docview.wss?uid=swg21293213

Technote 1089879
Description of "Server_MaxSessions=" and "Server_Session_Timeout=" Parameters
http://www.ibm.com/support/docview.wss?uid=swg21089879

Lotus Domino 7 performance in production at IBM on pSeries servers
http://www.ibm.com/developerworks/lotus/library/domino7-pseries-performance/

Domino Configuration Tuner:
This setting is included in the DCT catalog of best practices.