One way to know about replacement is to check for water flow. If it decreases significantly, it may indicate clogging and contamination. However, according to experts, this method is not the most reliable one. You should typically change the filters every 40 gallons, or every two-three months. However, this also depends on your household needs, the hardness of the water, and unit type. If you have very hard water with large sediments, you should clean the filter weekly. Moreover, if you have a large family, you should consider replacing them every six weeks. Most modern brands and units come with indicators to alert users regarding cleaning and replacements. They let you know if and when the filters malfunction to prevent users from consuming contaminated water. You may also follow the brand’s instructions for more specific details regarding maintenance.