This varies from LED to LED and is used as a guideline rather than a guarantee . The shortest is currently 20,000 hours and the longest is 100,000 hours, although the Aurora M10 which was originally 100,000 hours but got reduced to 40,000 hours in 2014.
Due to the lower price of an economy GU10 LED, manufacturers can not afford to support longer life expediencies so reduce the hours as a result of costs rather than quality. A GU10 can easily be replaced whereas an integrated LED downlight would need to be fully replaced so without a longer life expectancy consumers would not feel confident enough in purchasing them.
To calculate how long an LED actually lasts for you need to consider how many hours per day it will be used for and divide this by the life expectancy. This will tell you how many days it will last for. Then divide the days by 365 and you will get the years. Here is an example:
The Philips 5.5W lasts for 40,000 hours. Divide 40,000 hours by 3 (hours per day). This gives you 13,333 days. Divide this by 365 (days of the year) and you get 36.53 years.
If you use it for 6 hours per day instead then it will last for 18.26 years. Alternatively if you had this running for 24 hours per day, 7 days per week it would last for just 4.56 years.
If you plan to run your LEDs for longer periods of time then ensure you choose a quality product such as the Philips Master LED range which is backed by a 5 year guarantee.