I have 3 daughters who are 7, 5, and 3 years old. I didn't allow them to watch TV till a year ago when I was talked into babysitting a large plasma tv while the owner left the country for a few years. Then it didn't take much to hook it up. I watched it for maybe an hour in total over 6 months just to remind myself why I got rid of my own TV in the first place.
However during those months my daughters deteriorated. Without TV it was possible to engage them - the two older ones were interesting to speak with. The 7-year old used to pester me for math problems subtraction, addition, and multiplication and actually kept a journal since she was 6. The 5-year-old knew the ABC and counted to 100 as well as tried writing phrases.
That was before. After 6 months, they became much more confrontational and rude - behavior learned from children's shows that seem to feel it's critical that kids never question that their parents are stupid, useless, and an object of ridicule based on the cartoon 'storylines'.
They started imitating the cartoon characters, who taught them that learning is overrated, that cool people talk in broken slang, that there are no morals to govern one's behavior but - 'I want it, and I want it now, and I don't care about anything or anyone else'. On the 6-months anniversary of plugging it in, my 7 years old told me that reading is stupid and a waste of time. That night, the plasma had an unfortunate accident (sorry Mark).
The first 3 weeks without TV again, was like watching one of those shows where crackheads or dope fiends in rehab go through withdrawals. My girls didn't know what to do with themselves. Moody, tears, threats, first temper tantrum from the oldest. Now they partially recovered but the damage has been done.
Books are shunned now, 6 months later, as something stupid, and there are frequent reminiscence sessions where the girls sit and talk about how awesome a tv is and how wonderful the programs are that show you everything instead of having to make pictures in your head and read stuff.
----Advice: too late for me
TV is what they indoctrinate you to mold you into a happy little bot incapable of independent thinking and always hand in hand with the popular culture and whatever that clique happen to be supporting and opposing that week.
If you want to grow vegetables, programmed to look down at you, then agenda-driven TV is the way to go. My advice - throw it out. There is plenty of stuff on youtube and other sites.
A 2010 study, University of London Institute of Education, [The consequences at age 7 of early childhood disadvantage in Northern Ireland and Great Britain], which used test results for 11,000 seven-year-olds tracked since birth (part of the Millennium Cohort Study).
"...watching fewer than three hours of TV a day (is) positively linked to the teacher assessment score." (p.19)
"...less than three hours of TV a day in the pre-school period are all linked with a positive trajectory in teacher assessment between the ages of 5-7" (p.20)
"Moderate TV viewing (between one and three hours daily) is linked to lower (social and behavioral) difficulties scores as compared to high levels of viewing (over three hours)..." (p.22)
(p.46)/(p.49) of the study are especially interest with the coefficients listings for various conditions affecting children of various races/neighborhoods/conditions at home/parenting styles. etc.. (including hours of tv viewing).
There is lots of good data here to go over, but its plain to see from the data on pp.46/49 that the less TV the better.
A 2013 study by the same team as Study-1 above using the same Millennium Cohort sample - [Social Class and Inequalities in Early Cognitive Scores] - published in Journal of Sociology
http://eprints.ioe.ac.uk/11611/ (docx download)
The study concluded that children of less educated mothers who watched 3 hours of TV each day were three months ahead of their peers (also with less educated mothers) who watched less than an hour of TV per day. Thus on average, children of less educated mothers benefit from watching television. For children with educated parents in stable homes, there is damage - physical health, mental health, progress, etc.