Establish fair and reasonable boundaries and then stick to them. X amount of time means X amount of time, no exceptions. You don't have to be mean but you have to teach them about rules. A lot of teens think rules are just glorified guidelines that don't really have to be followed. Sit down and brainstorm some ideas for activities your kid can do other than video games. Suggest some of them to see if they spark any interest. If your kid wants to be an ass and mope around all day or sit there like a schmuck because he doesn't have his game, well that's his choice. But once you establish rules, stick to them.