Large residential carbon systems commonly advertise multi-year or million-gallon life, but actual performance depends on disinfectant concentration, flow, water chemistry and whether the system is sized correctly.
Why gallon ratings vary
Manufacturers often publish capacity in gallons under defined test or marketing conditions. A household using less water can reach the time limit before the gallon limit.
Chloramine is harder on carbon
Chloramine reduction generally needs catalytic carbon and more contact time, so media life and sizing should not be extrapolated from free-chlorine service.
Prefilters protect the main media
Sediment prefiltration helps keep the carbon bed from loading with particulate matter and preserves pressure.
When replacement is due
Disinfectant breakthrough, declining taste/odor control or reaching the manufacturer service limit are signs to evaluate the media.