How long do LEDs really last?

I was contemplating my own mortality recently, when I realized 2015 passed by in a blur. Ruminating about the loss of yet one more year, I looked at my LED-laden lamp and thought, “Sure, you might last 50,000 hours, while my expiration date is virtually unknown.” But, was this true? Happy to take the thought process from my own shelf life to that of an LED, I began to wonder if that 50,000, 35,000, or 25,000 hours of use from an LED could possibly be accurate. For one thing, I don’t mark down the date of purchase for my LEDs. Do you?