Luxeon LEDs do not burn out, but they do grow dimmer over time. So lifetime is usually stated as the number of hours an LED can be operated before it's light output drops below a certain percentage of their original brightness.
In the case of most LUXEON LEDs, the manufacturer projects that LED will run for 50,000 hours before the light output drops to 70% of its original output.
This is assuming that the LED:
- Is powered at or below its maximum recommended current level.
- The junction temperature of the LED is operated below the maximum recommended rating for the LED.
Lumileds has published an extensive Reliability Data document that details Rebel lifetime projections and the test conditions used to produce this data.