When should I replace my grinder burrs?
Replace burrs when performance degrades, not on a calendar. The signs are a grind that suddenly needs to be much finer for the same shot time, visibly mixed dust and boulders, more heat and noise, and a loss of sweetness you cannot dial back.
Burr wear is gradual and it fools people, because you compensate a click at a time without noticing. The clearest objective signal is drift in the same direction over months: if a coffee you used to pull at a mid setting now needs a setting near the fine end to reach the same shot time, the cutting edges have dulled and the burrs are crushing more than shearing.
The second signal is the grind itself. Tip a dose onto a white surface and look. Dulled burrs produce a bimodal mess, a lot of powder alongside noticeably larger fragments, rather than an even texture. In the cup that reads as muddy and simultaneously harsh, and no grind setting resolves it because the distribution is the problem.
Third, listen and feel. Worn burrs work harder: more motor noise, more heat in the grounds, slower throughput for the same dose. On a hand grinder it shows up as more effort and a gritty, uneven crank feel rather than a smooth one.
On lifespan numbers, be skeptical of anything precise. Manufacturers quote figures in the hundreds of kilograms of coffee for steel espresso burrs, with coated burrs quoted higher, but those numbers vary a lot by burr, by roast level and by how you count degradation. What is safe to say is that a home user grinding a kilo or two a month is on a multi-year timeline, and most home grinders are sold or replaced long before their burrs die.
Before you buy new burrs, rule out the cheaper causes. Stale coffee oil caked on the cutting edges mimics wear exactly, so strip and clean the burrs first. Misalignment mimics it too, and shimming a misaligned flat set costs almost nothing. If both are ruled out and the symptoms persist, then replace, and replace as a matched pair rather than one half.