My oldest has ADHD so from about 20 months old the only way I could get her to sit still long enough to let her body have a chance to fall asleep was to stick a Dora dvd on (no abc2 back then). Dora was my hero! Lol
Some days it took 1 episode, some days it took 5.
Then I started putting on the wiggles or high 5 while I was cooking etc.
She seriously just did not stop and toys and other activities just didn't hold her attention, yet tv would.
Now she is almost 9 and really doesn't watch a lot of tv. She'd rather be reading a book and is an extremely intelligent little girl! On weekends if we aren't going anywhere she might watch a total of 2 hours a day, and that's usually a movie that I will watch with her at night.
So the amount of tv she watched when younger really hasn't effected her.
My middle child has never watched as much as she'd rather play with her toys or be doing craft.
My youngest was never interested in tv until age 3 and at about 3 1/2 became a bit of a tv junkie, but again he has ASD and ADHD and there aren't many activities that hold his attention.
So all my kids have been different and I don't think there is a right answer.
As long as they have plenty of opportunity to be trying other activities and to play outdoors.