That is one of those things where it is difficult to answer, because the diamond tip is worn down based on the hardness and abrasiveness of the stone / metal /plastic you are engraving. It’s like asking how long to car tires last. Do they last months or years or decades? Well, just like car tires, they last so many feet of engraving, based on the above info.
So, no one really wants to say how long they last, because I don’t think most programs track linear distance for each file, and I sure don’t track total linear distance on my DDE bits.
What’ll happen when it starts to get dull is the lines get lighter, and you have to push down harder by increasing the plunge depth. It’s a gradual process, so unless you abuse the bit and rip out the diamond tip, it’ll just eventually get lighter until you replace it. It’s not just - one day it doesn’t work. You’ll have time to replace it once you notice getting lighter marks.
Here’s some of the things I’ve done on my SO3.