I'm not an expert, but here's a link to a forum with lots of experts. Consensus is that they will lose half their brightness over 60,000 hours.
There has been much misinformation about the longevity of today’s plasma TVs. Like all other display devices, there are two parts to a television. The first is the internal electronics. Modern circuit designs are extremely reliable and all televisions, including plasma’s circuits, are designed to provide extremely long life. The second part is the section of the television that produces light.
In plasma, it is the phosphors within the glass panel. Phosphors are used to produce an image in standard picture tube (CRT) televisions as well, and in three-CRT rear projectors.
Television manufacturers base the projected life of all TVs on “half brightness,” meaning the time it takes for the display to create an image that’s only half as bright as when the TV was new. The three-CRT rear projector is generally rated at 15,000 hours until half-brightness. Direct-view CRTs are generally rated to 30,000 hours. Panasonic plasmas have a half brightness rating of 60,000 hours – four times the life of CRT rear projection. This equates to more than twenty-three years at seven hours a day viewing, around the average daily TV viewing time per U.S. household. With its inherent high brightness, a Panasonic plasma will likely retain its image quality for many years.
The AVS forum is the best site by far. I set up my projector and sound system off that site