They're supposed to last several months and up to 15,000 gallons, but on our well water with a pre-filter cartridge, they only lasted a week. We usually use Culigan D10s which are good for 250 gallons and we get 3 to 4 weeks on one D10 cartridge so I think it's all hype and pure junk. Don't waste money. Luckily they have free returns so we will refund them.