A filter has a capacity and a service interval. Replacing it late can reduce performance, while replacing it early may waste money without improving the result.
Start with the manufacturer’s guidance
Record the installation date, model and stated capacity. Intervals may be based on months, litres, pressure drop or a combination. Follow the instructions for the specific cartridge and housing.
Watch for changes
Slower flow, a return of taste or odour, visible sediment, leaks or a service indicator can all justify checking the cartridge. Do not rely on taste alone for safety decisions.
Match the filter to the problem
Carbon, sediment, specialist media and reverse-osmosis systems address different objectives. A filter cannot automatically solve hardness, and a softener is not a substitute for every filtration need.
Compare filter types and softeners versus filters before committing to ongoing replacements.