Run the server settings editor or edit server_settings.xml. Set Session timeout to desired value. All idle users will be disconnected when idle period > session timeout. If a disconnected user tries to do something with service desk he/she will be automatically reconnect.
From your application server, select start, programs, hp openview service desk 4.5, application server, and select open the server settings editor. The section for the session timeout entry is on the general tab.