The LED doesn’t burn out in the conventional sense. To have a way of describing what happens, product life is measured as lumen depreciation. The life span of an LED is much longer than that of incandescent, fluorescent or HID lamps. Fluorescent or HID light sources also show a reduction in light output before they fail completely.
The concept of lumen depreciation is not new. The Illuminating Engineering Society of North America (IES) current standard for calculating the life of an LED is the operating time at which the LED light output is reduced by 30%.