I was doing a read for a project I'm doing and I found out that Sygate Personal Firewall has an option for parents to limit internet time. I thought I'll give you a link if you are still looking.
http://download.com.com/3000-2092-10184 ... ag=lst-0-1
P.S I was 16 two years ago, and I probably wouldnt like it if my dad limits my interent. But, if helps your job as a dad.