Every hour of TV watched reduces your lifespan by 22 minutes
Want to Live Longer? Turn Off Your TV Time Magazine
This was a study of 11,000 Australians in 2008
A previous study in June 2011 found that for every 2 hours of additional TV watching there was a 13% increase of dying in the next 7 years
Possible reasons which they listed: lack of exercise, tendency to eat junk food
Note: Disadvantaged and people who have not finished high school watch TV more and do not live as long.
VitaminDWiki adds a reason: By using TV/video games/multimedia not get out in the sun as much.
Would be nice to find a study of how much vitamin D levels decrease with hours of watching TV.
Previous News item on perhaps the same study
Does watching TV really kill you? Jan 2010

See VitaminDWiki
Disadvantaged get less sun and have poor health in the North– Aug 2011
Study quantified the problems of increased TV watching by toddlers – May 2010
That web page has the following graphic
Sanity check
5 hours of TV a day = 1825 per year = 91,250 per 50 years
20 minutes = 1/3 hour.
1/3 * 91,250 = 30,416 hours = 1267 days = 3.5 years: possible