Honestly.
My kids are 10 and 12. I know it's the holidays, but all they seem to do is move between the tv, Xbox and the laptop. If I kick them off one thing, they go and sit in front of one of the others. From pretty well 8am to 9pm, apart from breaks for meals and the odd half hour that I throw them outside to play.
Do you set limits? Or are you laid back about it?