Start with the product documentation
Use the cartridge maker's applicable service interval or rated capacity for the actual configuration, not a generic number from another system.
Household use matters
A larger household or higher water demand can reach a rated capacity sooner than a lower-use property.
Watch for changes
Changes in pressure, flow, taste, odour or visible condition can justify inspection. They do not prove a specific contaminant result.
Keep compatibility recorded
At handover, record the APF, SCF and CCF model details, safe replacement process and who to contact when the installation needs professional attention.