It will be extremely rare for anyone in this group to ever actually find the hardened flat or conical plates/burrs in their grinders have become worn and need replacement.
This is simply due to conditions of little real use in the domestic situation.
And even in business / commercial situations most wouldnt recognize when plates require replacement, and the "behaviour" of the grinder in such a circumstance may well vary from model to model depending on the type of burrs/plates used and the amount of grinding / work done.
Symptoms may include settings becoming generally "finer" over a period, but in terms of the humble cafe grinder (most of which in real life are never adjusted over considerable periods so no one would know the setting has been getting finer and finer on the scale) , the usual story is to arbitrarily renew the plates when the grinder starts overheating and cutting out during peak periods. If you place your hands around the adjusting ring at the time the grinder stops, it will feel so hot as to almost burn if you leave your hands there. This is heat from friction from worn plates.
If you were keeping a log, you could then work out how many kilos of coffee have been ground by those plates....
Some "experts" will advise you to change the plates when arbitrary useage figures have been reached, but that does not account for different make & model grinding plates made of different grade materials or having different types of hardening.....besides this can be directly affected by the type of coffee beans/blend density in use. So we dont think much of arbitrary useage figures except maybe as a reminder at that point to be on the "lookout" in case the plates might be in need of replacement somtime soon.
And any difference in the brew as a consequence of having used worn or new grinding plates, in real terms will be very hard to pick indeed and I sincerely think that very few would be able to tell the difference except where two side by side grinders might be used to make a direct comparison, one with new and one with worn plates...this could be the subject of a future coffeesnobs project...to prove or disprove that anyone might be able to tell the difference in the cup in real life!
If you place a set of clean worn plates next to a set of new ones, you will be hard pressed to see any reason why one set might be "more worn" than the other, so you cant rely on finding visible wear patterns Im afraid!
This Im afraid, is not an exact science!