Keep Session Variables Active for longer period in ASP.Net

Sir,

• I have made login page.
• When user log's in with it's emaild and password, I have stored it's email id in session variable as: Session("user").
• After logged in, If i leave the page kept opened OR ideal for sometime and after thatt if I select any option from user profile, the session variable get expired and send's me back to login page. 
• I want that if I would create 2 to 3 session variable's it should remain activated for longer period. So that even if page kept ideal for longer period and then if selects any option, the session variable's should not be expired soon.

Please do the needful sir, It's giving me lot of problems..