It's an estimation. For example, here is an article on plasmas:What does 60,000 panel life really mean?
I've noticed a lot of LCD TV panels also making this boast. To me, though, it means that the Panel/TV will probably last as long as a CRT TV. From the article:
"This is a longer life than a tube based television."
In my short 48 years, the average color CRT lasted about 10-15 years. Plasmas, evidently, started off with fairly short lives. So, I think the statement is more of a statement about the longevity of the TV set... that it will last as long as CRTs of previous generations.
I am simply hoping for reasonable longevity. 10 years would be great, 20 years would be better (at less than 4-6 hours per day).