The unit comes with a measuring gauge, which you use to measure the diameter of the ceramic filters. If at any point the measuring gauge slides over the ceramic filter media without being forced, the filter needs to be replaced. It is recommended you replace the filters after six months of continuous use, and since this is a high capacity system, you can expect up to 50,000 liters/13,000 gallons of pure water during that time. The filters can be cleaned with the supplied cleaning pad as well, which you do once you notice a drop in flow rate. You can do this several times over the life of the filters.