They load up long before they wear out. Ceramic is extremely hard, so the abrasive surface lasts a very long time, but it clogs with steel particles and stops cutting. A gray rod is not worn; it is dirty. Scrub it with an abrasive cleanser and it comes back.
The gray or black streaking on a used ceramic rod is swarf, the fine steel removed from your blades, packed into the surface. Once the surface is full it slides rather than abrades and the rod feels useless.
Actual wear does happen, mostly as flat spots where you always work, and it takes years of regular use to matter.
Warm water, a scouring powder or bathroom cleanser with a mild abrasive, and a scrub pad. Work until the white comes back. Some people use a rubber eraser block, which also works.
Do this every few sharpening sessions rather than waiting until the rod stops working entirely.
If your rod has flats or a triangular profile, rotate through the faces so wear spreads evenly. On a round rod, vary where along its length you work.
Ceramic is one of the better choices for maintaining high-carbide steels short of diamond. See grits.