We recommend changing a filter on an annual basis or when the flow rate becomes inconveniently slow. Our water quality is changing all the time.  Even if you have your water tested, that is just a snapshot on that particular day, so don't be surprised if the frequency of cartridge change fluctuates from one year to the next; this is normal.    What matters most is knowing that regardless of the water quality going into the water filter, the quality coming out will always be the same.  This is the promise that Everpure delivers on and that is why it is so important to only buy a water filter that is third party tested and certified by NSF. Without NSF, the manufacturer can make whatever claims they want. Read more on this important certification.On the residential filters, an eleven (11) month countdown timer is included. We will also include a date sticker, with your order, to affix to the filter upon installation.