MSN Messenger Time Limit-doable?

I have a very greaty computer hogging child who will not stop with the msn messenger'ing. She's constantly on it and get's one sentence of homework done in 5 hours comp time. I need to know is how to limit the run time of msn messenger. I can't seem to find anything, I've try googeling, and using firewall rules, but none have time limit specifications available. I don't want to stop the program entirely as communication as a teen is important, but can't stand that she get's consumed by it! I also don't want to buy anything, and if avoidable I dont' want any parental control programs and that junk. Maybe just a time limit program or add-in if anyone knows of something. Suggestions????

All I want is to set a time limit for say 1 hour per day that msn messenger can run on the comp. can this be acheived through msconfig or something???

Any and all thoughts appreciated.
Thanks