It depends on the hardness of the site water. The Water Treatment Capacity table in the HYDROFILL Technical Brochure gives a range from 500 gallons, for extremely hard site water (greater than 250 PPM of hardness), up to 3500 gallons for site water that is less than 60 PPM hardness. Those numbers are estimates for the large cart-mounted 4-bag unit; the 2-bag unit will do half that. Conditions vary a lot, so use those numbers as a guideline. The important thing is to keep an eye on the PPM meter on the outlet of the HYDROFILL. Once the resin bags start to deplete, the PPM value will start increasing. When the value reaches whatever maximum PPM is specified for the system being filled (typically 30 PPM to 100 PPM), then it's time to replace the bags. You can extend (a little) the life of the resin bags by slowing down your flow rate through the HYDROFILL, giving the site water more time to interact with the resin beads.