I don't have much experience with them except that I just bought a boat with one and while it was on the hard I wanted to replace the bellows. I talked to PYI? and they suggested that I replace the carbon block also since it was pretty old. I compared the old one to the new and the wear face on the old was considerably wider (worn) than on the new one. So I would say that they do wear, just don't know how many hours they are good for. I think the one I replaced had been on the boat since new in 88, with 4400 hrs on the engine but I am not sure. They told me I could dress the face of the SS ring with 1200 grit wet sandpaper, which I did. I also compared the new and old bellows, the old one was permanently compressed by about 5/8" so it was not putting as much force on the seal as it was supposed to. They recommend replacing bellows every 7-9 years if I remember correctly. Give them a call, they were very helpful, can probably tell you what's going on.
SV Laurie Anne
1988 Brewer 40 Pilothouse