Quick take
Expect a properly sized whole-house catalytic-carbon system to last years, not months, but do not rely on a gallon claim without considering chloramine concentration and real service flow.
Why chloramine changes the calculation
Chloramine is more persistent than free chlorine and generally requires more catalytic activity and contact time.
Media volume matters
A larger bed operating at lower flow has more contact time and generally more treatment capacity than a small undersized tank.
How to know performance is declining
Disinfectant breakthrough at the outlet is the practical sign that media performance is no longer adequate.
Testing beats guessing
Simple chlorine/chloramine residual testing can help track system performance rather than replacing media blindly.