How much X-Box live is too much?

My 15 year old stepson is at our house every other weekend. As soon as he gets the chance he darts upstairs and starts playing x-box live, sometimes for a stretch of 12 hours, into the night, only breaking for bathroom, drink or junk food to munch while he's playing. He gets very aggitated when interrupted. If he has a friend over he will always be the one playing and the friend is standing by watching. Needless to say, he is always trying to find a different friend to stay over. I think they are getting bored with watching him play. His father, my husband, also made the mistake of making a nice little set-up for him with a 52 inch tv in his room."BIG MISAKE" The boy's weight is following his mother's side of the family, which is not good and he barely moves around anymore. I tend to steer away from any kind of discussion with my husband about him...he thinks I am always picking on him. But the boy is spoiled, selfish, self-centered, a show-off and bossy with his friends....and his dad does not see it. He caters to him. Any suggestions?