If User idle for some time log off automatically

Generally, if you're using Session in your application and, if the user's session is expired, and if the user is trying to do something, he will be redirected to the login page. 


You can the following links.
Don't show the dialog, automatically the user will be redirected to the login page. 

http://programmerramblings.blogspot.in/2009/08/aspnet-session-timeout-control-jquery.html
http://blog.oscarscode.com/dot-net/display-session-timeout-warning-with-asp-net-ajax/