There are three main factors that affect how long a water filter cartridge will last:1. Your local water quality (which can change)2. Your family's consumption3. The water pressure coming into your homeYou want to be open to the concept that these three variables can change, especially your incoming water quality and consumption.  You could drink less but the water is dirtier than usual and therefore experience shorter life.  You could drink a lot more but the water is a lot cleaner than normal and experience longer life.