Script Summary: Facebook addicts, limit your Facebook visits ! This script limits frequency (for example allow a visit every 4 hours), and the visit duration (for example a visit can last a maximum of 20 minutes).
When you are on the website, LimitFacebook displays the time of your next "self-allowed" visit in the address bar and in the status bar.
To customize frequency and duration limits, simply right-click on the greasemonkey logo in the bottom-right of the window, and choose "LimitFacebook Preferences" (to display this menu, your current tab has to be on facebook or about:blank).
Actually this script could work for any site, simply change the url if you want.