Good quality LED lamps can have a lifetime of 50,000 hours or more but the average is around 35,000 hours. But this all depends on their design and how they dissipate heat from the LED chip. Depending on how many hours they are used a day, that can last as long as 20 years.
Unlike other lighting products LED lamps do not completely fail - they get dimmer. At 50,000 hours of use (or their rated lifetime), an LED lamp will be designed to provide at least 70% of its initial light output (LOR). The lifetime is dependent on many different factors including lamp design, LED chip type, ambient temperatures and fixture design.
Actual life of an LED is complicated to calculate and should be based on LM-80 measurements (calculated by independent testing labs) of the LED sources. Well-designed LED lamps try to get the optimum balance between light output and longevity.