What a lifespan rating means is that a Plasma set will lose approximately 50% of its brightness during its rated lifespan time. Based on even the modest early 30,000 hour rating, if such a Plasma television is on for 8 hours a day, its half-life would be about 9 years - or, if on 4 hours a day, the half-life would be about 18 years (Double these figures for a 60,000 hour half-life).
However, with some sets now rated at 100,000 hours, this means that if you watch TV 6 hours a day, you will have an acceptable viewing experience for about 40 years. Even at 24 hours a day, a 100,000 hour hour half life is still about 10 years.
Keep in mind that, just as with any television technology, display lifespan can also also be affected by environmental variables, such as heat, humidity, etc.. However, in most cases, a Plasma Television can provide many years of satisfying viewing.
Keep in mind that a standard television loses about 30% of its brightness after about 20,000 hours. Since this process is very gradual, the consumer isn't aware of this effect, except for the need to periodically adjust the brightness and contrast controls to compensate. Although the performance of individual Plasma televisions can vary, overall, as a product class, a Plasma television can deliver many years of acceptable viewing.