Filter cartridges have a finite capacity, measured in litres, and they stop working when it is used up.
The intervals
Jug filters — 4 weeks, or around 100 to 150 litres.
Machine cartridges — 2 months, or a stated litre count. Most machines track this and tell you.
Plumbed carbon blocks — 6 to 12 months depending on capacity and usage.
RO membranes — 2 to 3 years; the pre- and post-filters far sooner.
Why not to stretch it
It stops removing anything. An exhausted ion-exchange resin is saturated and simply passes hardness through.
It can release what it captured. Saturated carbon can shed trapped contaminants back into the water.
It grows bacteria. A damp carbon block with no chlorine is a good environment. This is the main reason for the schedule, and it is why intervals are stated in weeks as well as litres.
Hard water shortens it
Capacity is consumed by what it removes. In a hard-water area, a cartridge rated for 150 litres may be exhausted at 100.
The signs
Chlorine taste returning. Scale appearing in the kettle again. Slower flow through a jug filter.
Do not wait for these — by then it has been ineffective for a while.
The machine setting
Tell the machine you changed it, and check the hardness setting is right. A wrong hardness setting means the descale counter is wrong too.
Last reviewed 8 August 2026