It is a lot of the first, and a little of the second. It is sort of like miles on a car. Plasmas are currently regarded as lasting longer than LED TVs, but both will last you a long time. For either, the genrally regarded "life" is somewhere from 50k-100k hours, plasmas being on the higher end, LED generally the lower end. On top of this, the "life" number given is actually when the TV reaches half brightness. It is still completely watchable, just not as nice as the day you bought it.
This means even at the low end, assuming you couldn't stand half brightness, and watching for 2hrs every day the TV should last you almost 70 years. You can even extend this by lowering the brightness from full, but honestly, with mustachian watching tendency, your TV will probably outlast you.
Edit: I saw I was getting ninjaed by tkaraszewski, but figured I'd post mine anyway.