I've had one, a 3" X 8", for about 15 years, and it's only slightly worn. If it was paper, I'd say about a 400 grit originally.
I've sharpened knifes, chisels, gravers, and even used it for the finish draw filing on a piece of barrel once. The way it's going, it should last another 15 years.
The brand name is EZE LAP Diamond Stone, Carson City, NV.
John