For comparison, General Electric lists a 30,000 lifetime for its T5
fluorescent bulbs until output declines to 80% of original. And they
provide lifetime output curves to show you how the decline in output
will occur.

White LEDs are actually ultraviolet LEDs with a phosphor coating which
converts the ultraviolet energy into white visible light. So, most
likely, they will decline in output in a fashion very similar to
fluorescent light bulbs, because they are both using the same method
of converting ultraviolet radiation into visible light.

.....

Jeff Walther "

That's not me; that is the guy that posted the comments on his site.

Update: I called Cree. They were very nice. Transfered me 4 times up the chain till I got a very knowledgable, nice person. He said that the Cree LR6C go up in lumens for the first 1000 hours and then start a slow steady decline in lumens until it reaches about 70% of the 650 initial lumens in (get this) 55,000 - 60,000 hours. That's 7 years if you leave it on 24 hours a day, he said; 30 years in a typical home. The world of lighting will be different in 20 years, so no worries if I want to change the bulb in 20 years :-)
The lumens quote CREE gives is impressive because they measure the output from the actual fixture, not the bulb which loses lumens as it travels out of and bounces around the fixture. Still considering the Genura, however. (and a TCP Springlamp).
Bob