How long is it guaranteed for?
Not a daft question, is seems they take 100 bulbs and power them up, when 50% have failed that is considered as life of the bulb, there has never been a guaranteed life, this gives us a problem, you buy as I did 16 bulbs and after 18 months 50% have failed and one would think hang on no where need the 10,000 hours it states, but without the other 84 bulbs in the group it means nothing.
Also how do you measure how long a bulb has been on for, with a standard lamp I could plug it into an energy meter so when the 8W bulb has used 80 kWh then it has done 10,000 hours, however likely the back-up battery in the energy meter will have failed well before that point. And even if it did last, by time it has clocked up even 40 kWh likely looking at over 3 years used 8 hours a day, do you really think you could return a bulb to
Screwfix saying this bulb only lasted 3 years it should have lasted 7 years, I want a refund?
First LED bulb in my house was likely 2010, so should expect failures around 2020 I do have a record of when early bulbs were fitted, after the CFL were such failures I expected same from LED, but has not happened, although they do give LED's a life expectancy, I have got red LED's in equipment which must be 30 years old and still going, the main component in electronic equipment that fails is the electrolytic capacitor, many bulbs have a capacitor inside them so looking at a 5 to 10 year life until the capacitor fails, this is likely what limits life of bulb.