Moen states that each filter cartridge filters about 200 gallons of water, which they say is about a 3 months supply. We feel that it largely depends on your water quality and we can't really give you an estimate. It is a good idea to replace your carbon filter often.
Bacteria can grow in a bed of carbon.
I want pure water. It depends a lot on your water quality. If you have bacteria in your water you might need an Ultra Violet light filter. The longer the contact time of your water through a large bed of carbon the better. Most filter cartridges that are on the end of a faucet spout do not contain a large amount of carbon.