L.M.
Any time that they are just sitting sedentary for any length of time should be limited. Sedentary would be considered tv, video games (on machine or computer), or playing with things such as putting puzzles together, legos, etc. Kids need as much exercise as they can get, and these days it hard to do. You can't just let them go outside without supervision anymore like we did when we were kids. And what parent has time to go outside to supervise them. We are all so busy these days.
Our son, age 7, is allowed very little TV/video game time during the school week. He might watch a little in the morning while getting dressed/eating breakfast. But, when he starts allowing the tv to sidetrack his morning routine, it's taken away. In the evenings, if he has had a good day at school and has all homework done, he gets to watch no more than an hour of "his choice" TV. How much is dependent on how much time we have in the evening before it bath and bed time.
On the weekend, we're a little more lenient, actually a lot more lenient. We make sure he has active time either indoors or out, but we really don't limit how much time is spent with the TV on. He's into Star Wars right now, so he'll turn on a movie and get his SW gear out and play along with the movie. When playing video games, if it is "sit still" games, he is allowed only an hour per day on the weekend. If it is a Wii move around game, he can play as long as he wants.