This is a common question among people who use Christmas lights for decorating their homes and Christmas trees. But as you can imagine, there is no clear cut answer to that question.
Fairy lights can be used for many years but occasionally you will find that a bulb blows and the entire set will stop working. You then have the task of finding which bulb is at fault and replacing it with one of the spare bulbs the suppliers include with the lights. Remember these lights are only for use indoors!!
LED light bulbs can also blow but due to their better design the rest of the bulbs on the set will still work. The damaged bulb cannot be replaced in the same way as with fairy lights and if a lot stop working the only option is to buy new lights.