Understanding the known factors of LEDs to help evaluate LED system lifetime discussed in this white paper from Lumileds. Link here (PDF)
One of the strongest propositions of power Light Emitting Diodes (LEDs) is their long lumen maintenance their ability to continue producing light output for many years of use, in contrast to most conventional light sources, which force users to go through repeated and frequent failure-and-replacement cycles. The market perception of an LED’s reliability is reinforced by the widespread practice among lighting manufacturers of offering long warranties on their LED luminaires.