You can do that by
1. Replacing the timeout value in web.config
<sessionstate timeout="40" />
here 40 means 40 minutes.
Or
2. In
Session_Start method of
Global.asax file, you can set the timeout property
Session.Timeout = "40";
Update:
If you want unlimited session then you may consider using cookies instead of session as the purpose of using session is bit different than your requiremnet.
Follow these useful links
unlimited session timeout?[
^]
Keeping ASP.NET Session Open / Alive[
^]
how to make login session unlimited?[
^]
Hope, it helps :)