To distroy a session variable

Dear Sir/Mam

           
            I have one login page and After log in successfully. I reachend a welcome page. On that page i want distroy that session variable after some amount of time.
Between that amount of time if he click anywhere of the form the session distroy just like a bank site or railway reservation on doing transaction. Otherwise user can move to further page. How can i do this? Please reply me asap.
 
Thanks a lot
Sanjay Kumar Gupta