I looked at the bottom of my poles and spotted that the rubber had worn away so much that the metal tip was showing through. I had previously bought a bag of some 20-odd ferrules, and this marks the first change. I'm not sure quite when I started using the poles - some time around October 2024, so we can say that my first set of ferrules have lasted about six months. These ferrules are considerably cheaper than shoes!
As my mobility changes I can start to investigate whether other types of ferrule would give some advantages, but at the moment I have no need to change type. Here are a couple of photos.