I've been using LEDs now for the last 5 or so years. Claims of 25000 to 50000 are far short of actual runtimes I'v e been experiencing. While some of have been running for 3 to 4 years so far, several (10 to 15%) have died in under 2 to 3 years. (I have ~40 units installed)
Such a high failure rate after so few years. (25000 ~ 11 years @ 6hrs/day, 50000 ~= 20yr) leads me to believe if they continue dying at the same rate, the average on the group will be far, far lower than the stated 25000 to 50000 hours.
I'd use the warranty as a rough guide. The cheaper bulbs have very short warranties an you should expect them to not last much longer than the warranty period. It seems like CREE is one of the few manufactures offering warranties that approach the stated 25000 expected lifetime.