The half-life of a television is the time period that it takes to be half as bright as when you first bought it. Top name brand televisions are rated at 100,000 hours. Viewing your television everyday for 7 hrs a day your television would last over 30 years at the 100,000 hour rating before it was half as bright. If your television is not turned all the way up on full brightness it may actually last even longer than that as those ratings are with the television on "flame" mode.
If you are choosing between LCD and Plasma buy a quality brand (Samsung, Panasonic, Sony, Sharp, etc) and all of them will last. If you are choosing between plasma & LCD here is your guide. If you are placing the televison in a bright room, will use it for heavy gaming or as a computer monitor buy a LCD. If you plan on watching sports or other fast motion, are placing in a room without bright light and watching movies with the best black levels and truest colors buy a plasma. Burn in on modern day plasma screens is close to impossible to achieve in normal use. Plasma's do use more energy than a comparable LCD but in most cases this will amount to 3-4 dollars more a month.
I have a 50 inch plasma Samsung plasma as my primary television in my home theater. It is fantastic. I have a 37 inch LCD in my bedroom that I game on and occasionally hook up to my computer to. My plasma gets heavy use and will last me 15-20 years easily.
Much research and purchases and ownership of several different models.