Burr Changes Based on Volume
If you are brewing with blunt burrs, the extraction target will drop even lower. Scott Rao calls this the ‘extraction ceiling’. Over time, 21% extraction will start to taste over extracted, so you will need start targeting 20% extraction. If you find yourself aiming for below a 19.5% extraction target, we would suggest it is time to change your burrs. This is never cheap. EK43 burrs cost upwards of A$500 a pair and Mahlkoenig rate them to do 14,300lb (6,486kg) of ground coffee.
If you need a more practical guide, Espresso Parts have a helpful list here of manufacturer’s recommendations for the durability of common shop and espresso grinder burrs. After you have ground that volume, you can, of course, continue to use the grinder until you notice over extracted flavours at low extraction yields. Never consider changing burrs based on a specific time period — e.g., once a year. For example, a shop grinder on a busy brew bar doing 50 pour overs per day with 15g per serving will only get through 274kg per year.