well i was not sure as i wanted to use asp as well so that when the 20mins ran out then a prompt box would appear asking to extend session.
I hope i do understand fully what u are trying to achieve, but here is my advice.
A session by default, as dhtmlgod said is 20mins, you could increase it with this
session.timeout = 100
objects, setting the values of cookies to the users hard drive. Then whenever the user requests a page, check to see if the cookie exists and then create or extend the session based on that value