Logout Problem

In my application i validate user name and password from database and only 10 user is allow to access application same time.

if more than 10 user try to access it will show error page and if some user logout than only it will allow wating user to access web application.

but my problem is this ,wating user can only access web application when current user click on logout button because i worte my code on button click.

but what happen if current user close the web page  without click on logout button or if power Failure or some other problem,

what i want when user close application without click on logout button  or due to some other cause he will not click on logout button ,after some 5 min wating user allow to access web application.

thanks for your time.

pls help me