QuickTimeKirk has pointed you toward the Parental Controls for User accounts, but I don't believe you will find the answer there. (I have three boys myself and have explored questions like this for some time.)
In a nutshell, Parental Controls will allow you to block sites, or set time limits and hours for machine use, but unfortunately there are no options for limiting "time spent @ website.com".
My best advice, as a parent, is to setup the machine in a family location, set up any other Parental Controls as necessary, then monitor your child's use of the machine when they are at it.
We parents just have to be pro-active in the online lives of our kids. It's not something that we can rely on software to do for us, because sooner than you think, these kids will find a way to outwit the probrams that are designed to limit them.
I feel your pain. I hope this is helpful. I also hope that if someone else reading this thread has additional info or ideas, plese share them.