Session variables are stored at server side, by default Session timeout will expire after 20 minutes. We can change the ASP.NET Session timeout in Web.Config and code behind. In this example, i have changed the ASP.NET Session timeout value to 1 hour in the Web.Config
Here’s is the solution
<configuration>
<system.web>
<sessionState
timeout="60" />
</system.web>
</configuration>
Note: If you have increase ASP.NET Session timeout then you have to increase application idle timeout from Application pool, if Session timeout is greater then Application pool idle timeout then Session will be expire because Application is recycled before session timeout.