If your feed material is high purity Silver >99%, a cell will last a long time because the nitrate is regenerated at the cathode when Silver is deposited and it picks up another Silver ion from the anode. Only when base metals build up in the electrolyte, typically indicated by color of the solution, does the electrolyte get depleted because the base metals consume the nitric and leave less nitric to digest Silver at the anode. Back in the day of cheap nitric, cells were run with cemented Silver which had high purity and the cells ran long and produced a lot of Silver with little maintenance. Today running sterling through a cell requires adding extra nitric to keep the Silver content of the electrolyte up because the copper requires over 3 times the nitric as Silver to dissolve. So the nitric drops, the copper concentration increases to a point where the Copper is high enough in concentration to co deposit at the cathode and produce impure Silver.
