You can't measure how long a chair is occupied from network connections. If you send one text message at 11:00, then send another at 11:15, does that mean you were sitting watching the phone for 15 minutes? Same problem. It gets even more difficult with how computers are so willing to access the internet of their own volition, for updates and the like. The computer could wake up at 2am and eat all his daily internet "time".

If you wish to measure occupancy of the chair, something local on the machine would work better.

This isn't the answer, but might be related. It forces Windows to logout when the screensaver goes on.

If a local solution won't do, then you'll need one of those obnoxious motel wireless systems that lock you out until you explicitly login, then kick you out again after a time limit.