How to actually tell when a tumbling stage is done

Do not rely on the calendar alone, the standard day ranges for each tumbling stage are starting points, not guarantees. Pull a few representative stones, rinse them off, and inspect them closely with a loupe under good light. The coarse stage is done when edges are genuinely rounded with no sharp corners remaining, not just smoothed a little. Medium and fine stages are done when the scratches from the previous stage are fully gone under magnification, a stone that looks smooth to the naked eye can still show visible scratch marks under a loupe. Checking partway through a stage rather than only at the very end also lets you catch problems, like a stone that is dissolving or breaking down unexpectedly, before it damages the rest of the batch.