I'll give a "non-scientific" anecdote - my son is very active, probably has quite a few of the criteria for ADHD, has been diagnosed with sensory processing disorder. He pretty much didn't watch any telly until he was 2, mainly because he wouldn't sit still! After age 2 I wouldn't let the Tv go on the in the morning, only for a set amount if time in the late afternoon, when he was most hyper and it calmed him down. And not every day either. So maybe 5/6 hours a week in total?
BUt the thing is, he was hyper from birth, my MIL told me she'd never seen a baby so active, he was always kicking his arms and legs about and walked at 9 months. He was BORN that way.
Now my neighbours kids all watched a LOT of telly from birth, I remember when DS1 was small being shocked that her daughter (the same age) was plopped in front of the TV watching baby Einstein while her Mum cooked dinner. She had favourite shows before 1! But all 3 of those kids are still placid and good-natured, they were born that way too. Of course they had lots of non-Tv time, and LOTs of social interaction, their Mum is always out or having friends over.