A DLP (digital light processing) lamp is the main source of light in a rear screen projection television set. DLP lamps do tend to age after a few years, with the length of time depending on the brand used and how many hours you have your TV set on. The more the lamp is used, the more the mercury vapor arc tube's electrodes wear out. What you'll notice is that your picture is dimmer or that you don't get any picture at all. The good news is that you only have to replace the DLP lamp, not the whole television, and it's something you can do in about 5-15 minutes.
Regarding how long DLP lamps last, as mentioned, it depends on the brand of the lamp. Philips and Osram are two of the most dependable brands, and you don't want to buy a remanufactured lamp. While the older DLP lamps last between 1,000 and 2,000 hours, the newer ones can give you 6,000 to 7,000 hours worth of TV viewing. How many years this actually translates into depends on how many hours a day you sit in from of your TV set.