On average, about 30 minutes for our 4 year old.
That means a lot of days he doesn't watch any TV...and some days we watch movies for 2 hours. Or a few episodes of Justice League or something.
What really bothers me is when he watch TV when I'm talking to him. Like...he'll walk into the room...a dumb commercial is on, and he won't look at me. It's like he's a TV zombie or something.