http://us3.php.net/manual/en/ref.session.php

When you create the session, add your own variable called 
$_SESSION["last_page_load_time"].

if ($_SESSION["last_page_load_time"] < time() - 20 * 60)
{
	// don't time out, keep the session going...
	$_SESSION["last_page_load_time"] = time();
}
else
{
	// delete cookies
	// unset session variables
	// destroy session
}

The above will completely control the 20-minute window from your end. Not the 
best, I'm sure, but it'll work every time.

~~~

As a side note, I'd encourage you to read Jeff Atwood's article on sessions.

http://www.codinghorror.com/blog/archives/001100.html

Matthew Gonzales wrote:
>  From my understanding of session, the session will be destroyed if the 
> browser is closed or the user initiates a command to destroy it. (a 
> button "log out"). Is there a way to set a time variable on sessions. 
> Say to 20 minutes of idle time or must a user log out?
> 
> Matt G
>