Is there any application that can block usage to a Mac after it has been logged in for X number of hours?
This would be sort of like a "parental" type software.
Mac OS X 10.5 and later have so called parental controls built into the OS. Look in system preferences to find them. You can control both the hours the computer can be used, and the time. For example you could restrict access to 2 hours per day, and only between 8 AM and 10 PM.
You can even set up parental controls for one computer to be managed by another.