For DS8
It varies quite a bit depending on what we're up to that day. On average kiddo is "plugged in" about 4 hours a day on days we're home.
It's important to note, however, that we homeschool... so kiddo isn't *gone* 8+ hours, and then plugged in the entire time he's home. Instead (on days we're home)...
He's allowed electronics time (computer, xbox, netflix, dvds) after all his chores are done in the morning until school starts - typically about 2 hours-, as well as some time during school and some time in the afternoon & evening. We don't have "set" times. We found when we did that he would struggle to "fill" that time, even if ordinarily he'd be doing or wanting to do other things. Instead, having a more flexible outline of after chores before school, and with permission / pop off when it's time to do other things just works better for us in our situation. Because while the average time when we're home is about 4 hours... when we had a limit, on days where we're not home he either wouldn't want to leave (to fill his allowed time), or he'd struggle to fill it when we got home. This way there are days with no time, days with 4 hours, and the occasional marathon.
And the 2nd thing to note is that 3-5 days a week in the winter (Nov-Apr) we get up at 5 and spend all day snowboarding and doing school on the mountain. Summer it's about the same but water sports. And on days where we are home we're physically active 4-8 hours a day.
SO FOR US: Apx 4 hours on days we're home is good balance between intellectual & physical pursuits and electronics time.
How it affects him: It's mostly positive. Kiddo is really into videogame design and filming, and both help develop that. It cracks me up as he watches a scene a gazillion times to figure out the flow or things that make a good scene/ transition into a different kind of scene. There's also the "shift gears" / relaxation aspect, as well as the "gluing his daredevil bum to a safe happy chair for an hour or two" aspect, which, in our ADHD household is a precious as gold. Conversely, on yucky days, the Kinnect helps run out the crazies when we're stuck inside due to bad weather (also gold... it's nice not to find kiddo 10 feet up in the air stovepiping the walls in the house, or doing backflips off the furniture). The interactive component on xboxLive is 50/50. On the one hand, there's great feedback/user content (kiddo can be doing photography or designing on Forge and be testing out what people like and why. On the other hand he'll get in tiffs about jerks (rare). The *educational* aspect of electronics (tv, internet, gaming) is something that is *off the charts amaaaaazing*. Far, far too much to go into detail here (from edu software, to live uplinks to archeological digs, to virtual museum tours, to gutenberg & TED... the list is hundreds of reasons long... so I have to stop).
If there's any negative (from time to time) he gets banned for awhile. Which is a useful parenting tool... so even the negative ends up a positive.