The Timeout property specifies the time-out period assigned to the Session object for the application, in minutes. If the user does not refresh or request a page within the time-out period, the session ends.
Specifies the number of minutes that a session can remain idle before the server terminates it automatically. The default is 10 minutes. Session.Timeout has no hard-coded limit. Most Web administrators set this property to 8 minutes. It should not be set higher than 20 minutes (except in special cases) because every open session is holding onto memory. It should also not be set lower than 4 minutes because clients rarely respond within that time resulting in a loss of session state.
IIS 6.0: The minimum allowed value is 1 minute and the maximum is 1440 minutes.