The general consensus is that you will likely see 20,000 hours or so out of the plasma. That's until it reaches 'half' of original factory fresh brightness. The number is more or less accurate and doesn't mean anything in years until you compute how many hours a day/year you use your system.
If you watch TV 3 hours a day, 7 days a week, that's 21 hours a week, or about 1,000 hours per year. That means you will get about 20 years out of the plasma before it is HALF as bright as new on a 20,000 hour set.
Bump the watching up to 6 hours per day, and you lose half the time - ten years instead of 20. Bump the hour lifespan of the plasma up and you gain years.
Generally speaking, plasmas are considered to have about the same lifespan, if well taken care of, as your average CRT (normal) TV. So, replace your old TV with the confidence that your new plasma will likely last just as long as the old unit. If you are really concerned, you may want to look at LCD television as well.... if you can afford to.