"If you leave your TV on many hours a day a plasma will be bad in 6 months time, the screens have a 'half life' that goes down rapidly if used a lot."
Most plasmas these days are advertising a 60,000 hour lifetime, which would be 6.85 years if the tv was on 24 hours a day. Not a major problem. On good plasmas the burn-in problem is also fairly minor, although I still wouldn't suggest leaving a static image or 4:3 "pillar box" image on one for an extended period.
My TV will be a year old this Christmas. I did a lot of research at the time and screen burn in would have been bad for a TV left on sometimes 16 hours a day. Improvements in a years time could make a difference now.
This is from one article that has been updated;
SCREEN BURN IN
LCD technology is not prone to screen "burn-in" or "ghosting" (premature aging of pixel cells) due to the nature of the technologies “twisting crystals.”
With plasma displays, static images will begin to "burn-in," or permanently etch the color being displayed into the glass display element. The time it takes for this to occur depends greatly on the anti burn-in technology of the manufacturer. Recent improvements by plasma manufacturers have certainly extended the time it takes to burn in a plasma pixel cell. In the past I was concerned to place a DVD on pause 15 minutes. Now, many of the enhancements such as better green phosphor material, and motion adaptive anti burn-in technology are greatly reducing the risk of burn in. It’s gotten so much better that I don’t even worry about it anymore. In a new model plasma from any top tier manufacturer I would put “ghosting” estimates at an hour or more now (Ghosting can be “washed” out by displaying static gray material). Permanent burn-in I would put at more than 10 hours.
ADVANTAGE: LCD, but not as much of a concern as a year ago.