session expiration

Hi,

i would like to ask how to set the session expiration.

on my site when a user logs in, i assign it to $_SESSION['username']

and on each page i check if (isset($_SESSION['username']) if not i 
redirect it back to login page.

my problem is it seems like it expires very fast when there's no 
activity, how can i make it in such a way it won't expire unless the 
user logs out?

regards
Ron