Quick take
Use the manufacturer replacement interval and rated gallons as the baseline, then replace sooner when flow, chlorine performance or local water conditions indicate faster loading.
Rated capacity
Weddell publishes an approximately 8,000-gallon cartridge capacity for the Duo under its stated test conditions. Real life varies with water quality and shower use.
Do not wait only for visible flow loss
A chlorine-reduction cartridge can lose treatment performance before it becomes severely clogged.
Annual cost
Compare the cartridge price and actual replacement frequency with competing shower filters rather than treating the showerhead purchase as the full cost.
Why exact replacement matters
Use the correct Duo replacement cartridge so the system retains its intended flow and tested treatment configuration.