Just downloaded StayFocusd for chrome, shit's pretty hardcore. If you want to change the settings to add more time, you have to pass a test first. And even after that, it still tries to talk you out of it.
If you're using a Linksys router, it's easy enough to setup an access policy. Stick the sites you want in there, set the time restrictions, add the IP / MACs of what machines you want it to effect and you're done.