This depends on the type of equipment you have. If an aerator is present, then typically, it should be serviced every three months at a minimum. An iron filter or sulfur removing backwash valve should be checked annually. Water softeners should have salt and citric acid installed every 4 to 6 weeks. The citric acid is advisable to make the resin or product in the softener unit last longer. The salt, citric acid, and service can be done yourself or by a delivery service company. More sophisticated equipment, such as ozone systems or point of entry reverse osmosis systems, should have a service contract with a company well versed in these sciences.