I've got a few cobs that I've had as long as I've been smoking pipes (close to ten years), and I still smoke them. I doubt they are as durable as briar, but they are good for decades of service if you treat them halfway decently.
I don' t have any of the un-sanded, unfinished cobs like
@mikethompson used for his test. All mine are of the variety where the cob is sanded and plastered. They all eventually start to crack after enough smokes, and I fill the cracks in with a bit of pipe mud. None of the cracks go all the way through the bowl. They appear to be just on the surface and mainly cosmetic.
Of the 20 or 30 MM cobs I've owned, I had only had any issue with two of them. One of them is a model without a hardwood plug in the bottom. The bottom of the pipe because very spongy and weak under the center of the bowl, so I carved it out and made a hardwood plug myself out of a dowel rod. I enjoyed the process and still smoke that pipe. I had one other cob that started cracking apart. I think it was just a bad cob combined with my poor technique early on where I smoked it way too hot.