I have a little different take on this. My 2 girls, 11 and 15 self-moderate and I don't worry about their time; it doesn't consume them. My son, 17 will play endlessly if allowed. And, here it is, we allow him to, to some extent.
He is ADD (which translates to hyper-focused when it comes to things he likes) and cannot separate himself on his own. We use parental controls to make sure he's off at 10 at night.
Here's the thing. I talk to him a LOT about his game (World of Warcraft) and have decided there are many positives:
1. Many things he is unable to do in his real life (because of his ADD), he can do here: plan, carry out a logical sequence, have patience for a long term goal, practice difficult tasks, do boring repetitive tasks to achieve a goal, work together with team members, be a team leader, keep track of a myriad of details and execute them effectively, be an expert.
2. He is very social, has great friends (in real life). When I pick him up after school he is surrounded by an eclectic bunch of 5-10 kids (popular and un-) and they give him hugs as he leaves! Friends ask him for advice on personal issues. He goes out with his bunch from time to time.
3. He is going to be a counselor in training this summer (7 weeks!). A real honor and something he is looking forward to, despite the necessary hiatus from WOW.
4. He does not drive around, he's not out at all hours, he doesn't drink or smoke or take drugs, we know where he is.
5. He joins us for meals, DVD movies, outings, etc.
6. He has great, solid self esteem.
7. He does well enough in school.
So...this leads me to conclude that there may not be a pat answer for your kid. It is all about balance, priorities, and what this particular kid needs. And, trust your intuition. Sometimes what the experts say just does not fit your situation. Then you bravely chart your own course.
H.