We're not unschoolers here (child-led in many respects, eclectic, and relaxed, but not unschoolers), so our experience may or may not be helpful to you.
We talk to our kids about our concerns with screen time. Not in a judgmental way, but just a frank, "this is not the healthiest activity to choose" sort of way. We've talked about obesity, lack of activity, media addiction, mood disorders, brain development, just the gamut of stuff. But we also talk about film as an art form, awesome animation or investigative reporting. We don't pretend to know everything that's good or bad about electronic media, but we try to put what we do know, what we've heard, what we're concerned about on the table.
We don't have cable, but we do subscribe to Netflix and Discovery Education Streaming. We have some dvds, and we occasionally borrow dvds from the library. We own a Leapfrog Explorer and a few games. We'll youtube a few videos too. Dh and I try to make available video choices that are acceptable to us if the kids should choose them, so no SpongeBob, but we do have some Transformers stuff.
Then we just ask our kids how much time they need to watch, and then we set a timer. Typically, they ask for 10 to 20 minutes.When the timer goes off, they go do something else. Very rarely, they ask to watch a longer video like Star Wars IV or Finding Nemo. Sometimes, I "make" them watch more.
Usually, it's a documentary or something for a subject we're studying. (They love documentaries, so it's not like they are hating this choice).
Anyway, this is the balance that we have found in our family.