Page expiring after some time

Sir / Madam,

I am working on Visual Studio.Net 2008  framework 3.5  with (vb).

I have created a session variable, when user logs in successfully. But after login if page kept ideal for sometime session get expired, and automatically redirects to login page.

So, I want that after login page should remain  opened and session would never get expired until user clicks on Sign out link or button.

For this do i have to set anything in web.config file?