Probably about 2 hours. And often it's just "on" while they are playing. The 2 y/o will ask for a video, the 4 y/o will put it in, and then they wander off and play something. Or the 4 y/o picks something (eg, Phineas and Ferb) that the 2 y/o doesn't get so she is not exactly "watching" it but she's around it.
They may watch more during he week. They goes to my mother's and MIL's house while I am at work. Who knows what they do over there.
I really don't have issues with TV. Mostly they get into a phase where they want to watch Annie or Tinkerbell or something every day for a month straight. They set their own limits and get up and walk away when they are bored. If I ever see them getting lazy or getting that glazed look in their eyes, we turn it off and do something else. Mostly they pull away from it on their own before I have to do this.
I am pretty lax about TV. I just don't see what the big deal is. My baby sisters (twins now in their 20s) watched HOURS of sesame street (and then other things like MaryKate and ashley movies when they were older) incessently (sp?) growing up and they are the smartest people in my family... 4.0, graduating college with honors and all that. And they are very fit and healthy and always have been. No long term damage there!
I don't really limit the TV at all. Just not a big concern of mine.