Screen Time

Do you set any limits on how much screen time you and your family have?
I try to set limits on TV, computer and other devices that have screens, but it is getting harder and harder to do with all the gadgets around.
I don't have cable TV, so that helps, but I use netflix, and youtube daily. At least using those I have a *tiny* bit more control over what my kids have access to, plus pretty much no commercials, which is the thing I hate the most about cable television in the first place. (Yes I know there are PVR's but I still get commercials that way.)
There are some studies that show too much screen time is bad for kids. I have noticed in our house when the TV goes on it's hard to turn it off. It's always "just one more show" or "one more minute" and that leads to sometimes wasting the whole day or evening in front of the boob tube unless I strictly enforce the screen being turned off.
But sometimes it's really hard when they are watching something as great as the magic school bus, or another interesting documentary or educational show. Sometimes the things I remember the most were from documentaries I watched on television as a child. I know we need to get out and play outdoors, and read and colour, but sometimes time in front of the tube is just the thing my busy family needs after full days of play and work.