Apparently, MS may be adding parental timers to the 360.  So they can set a usage limit for a given time interval.  It's a great idea.  Just like the parental controls in the original xbox.  Unfortunately, they never marketed that functionality to parents.  I hope they do better this time should it turn out to be a feature really are adding.

http://www.engadget.com/2007/11/06/xbox-360-parental-timer-coming-in-december/