LIFE CYCLE. The industry standard measure of the life-hours for a product is to put 10,000 items in a room. Subject them to whatever is considered the "normal" off/on cycles, possibly accelerating the cycles. At the point where exactly HALF the bulbs have stopped working, THAT is the listed hour lifetime for that model.
A rep from American Fluorescent recently explained to me that it is a little different with LEDs. The element may run 30 years + and still be putting out light. But the elements do get dimmer as time goes by. Up to the stated life...let's say an item is an 8 year rated LED....the brightness is nearly the same as brand new. But then as further time goes by, it will get dimmer and after a certain period of time will be considered unsatisfactory, and that is when you replace it.
Yes, I do get to meet a lot of "reps" .. Long story.