It depends on the unit. On the one I linked it shows the useable life span of the different chambers. I also purchased a $15 TDS
meter to test the ro
water so that I know when it's time to replace them. The purewaterclub on ebay also sells the cartridges for all the units they sell. Some of the items even come with extras so you can have them on hand. Just check out that sellers other items listed and you can see.