Each TAPP 2 filter highly effectively removes the chemicals from 1200–1500 litres of water (depending on original water quality), the equivalent of 3 months of average household water usage. We recommend replacing the cartridge after 3 months even if you haven't filtered 1200-1500 litres. This is because the filters become saturated with contaminants over time and there is a risk of bacterial growth after 3 months. This happens with all activated carbon filters.