Session expiration in sharepoint 2010

Hi,

I am new to SharePoint 2010. I am working on web part based application. When SP user logged in with SP credentials, and user left browser idle for more than 20 minutes, and again if user try to access application, then SP should ask for relogin. But it is not happening. 

I have tried configuration of Office SharePoint Servers Shared Services section, clickConfigure session state through central administration but unable to get that path in central administration.

Thanks for replies in advance.