Based on our experience with incandescent bulbs, when we hear that an LED will last 25-50,000 hours, most of us assume that's when the bulb will burn out. However, that is not the case.
In reality, those claims are really arbitrary—no one really knows how to define the lifespan of an LED quite yet. That's because LEDs do not burn out like an incandescent bulb, rather, their brightness slowly fades. So, if the lifespan of your LED is listed at 25,000 hours, that is the point when your bulb will most likely be shining at around 70% capacity (the industry assumes people notice a decrease in brightness at that point). Some engineers have even suggested that lamps should be made that increase power to the LED to combat this problem—although that would tend to defeat the purpose of an energy saving bulb. It would also decrease the life span of the lamp.
So, the moral of the story is that manufacturers need to come up with a different system to accurately convey the lifespan of their products to incandescent and CFL converts. Personally, I don't think this is much of an issue. I would much rather replace a bulb after 50,000 hours because it got too dim then replace a traditional incandescent after 1000 hours with a bandaged hand because it blew out while I was chopping up something in the kitchen. [NYT]