Add
<sessionState timeout="540"/>
Also set the app pool idle timeout to required timelimit if using IIS 6.
The Timeout property cannot be set to a value greater than 525,600 minutes (1 year). The default value is 20 minutes.
Disadvantage: You will have performance issues if you have large number of users and with increase in session timeout, your inactive sessions will remain in Web server memory which may cause application pool to recycle, which would result in loosing all sessions for all users.