You'll want to update this on each page so that its not 15 minutes from login, but 15 minutes from last activity. (Which of course means that if for some reason you wanted to limit total login time, you just wouldn't do this next step).
Now finally.. in application.cfm or your header file or whatever you like (For those of you that aren't familiar, application.cfm at the beginning of each request meaning that if you type in a cfm page in your url, it will pull it up there but it will not execute it again before calling a custom tag or cfinclude since those are part of the same request).
Change loginidentifier to whatever your identifying variable is.. wheter it be session.logged or cookie.lastloggedin or whatever it is you use.
Change loggedinvalue to whatever value is specified for the loginidentifier variable meaning the user is logged in.