Some parenting experts argue very strongly that any screen time is bad for a child and that it should be at the least heavily limited and at best not allowed at all. But many parents I know rely on some screen time to break up the day, give them a few minutes respite to get housework done or as a really useful educational tool.
At the moment by eldest is only 4 so it's pretty much a self restriction....unless we're watching a movie all together he gets restless after about 30 minutes of TV so at that stage he'll just go back to playing with his toys. We're out a lot with various activities and errands so TV time is normally in the afternoon which gives him some chill out time and me a chance to cook dinner or tidy up. I'm sure as time goes on there will need to be some limits.
In the past, we didn't limit screen time in any way and relied on my son to self-restrict. However, we noticed that on days when he spent a lot of time on his iPad, his behaviour wasn't as good as on other days and he seemed unable to self-restrict enough to prevent this. So now we don't have a total ban on screen time, as I believe he learns quite a lot from various apps and videos, but we do limit the time he spends looking at a screen.