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Change a pitcher or faucet cartridge every two to three months, an under-sink carbon cartridge every six to twelve, a whole-house sediment cartridge every three to six, a reverse osmosis membrane every two to three years, and a UV lamp every year without exception. But the number on the box is gallons, not months — and the months printed alongside it assume a household that may look nothing like yours. Below is the full schedule by filter type, plus the one signal that actually tells you when a cartridge is finished.
The Schedule by Filter Type
| Filter | Typical rating | Typical interval | What tells you it is done |
|---|---|---|---|
| Pitcher cartridge | 40–120 gallons | 2–3 months | Slow drip through the funnel; chlorine taste returning |
| Faucet-mount cartridge | 100–320 gallons | 2–6 months | Flow slowing to a trickle; indicator light |
| Refrigerator filter | ~200–300 gallons | 6 months | Dashboard indicator; taste; slow ice production |
| Under-sink carbon cartridge | 500–22,000 gallons | 6–12 months | Chlorine taste returning — nothing else |
| Whole-house sediment | Varies with load | 3–6 months, less on a well | Pressure differential of 10–15 psi |
| Whole-house carbon | ~100,000 gallons | 6–12 months | Chlorine taste; the calendar |
| RO sediment and carbon pre-filters | — | 6–12 months | The calendar. These protect the membrane |
| RO membrane | — | 2–3 years | TDS reading creeping back toward the feed water |
| RO post-filter (polish) | — | 12 months | Flat or stale taste |
| UV lamp | ~9,000 hours | 12 months, always | Nothing. It still glows. Use a calendar |
| UV quartz sleeve | — | Clean yearly, replace every 2–3 years | Visible film or frosting |
| Softener resin | — | 10–15 years | Hardness returning despite correct salt and settings |
Why Gallons Beat Months
Every cartridge is rated in gallons because that is what it is tested against. The months printed on the box are that gallon figure divided by an assumed household — usually a family of four using filtered water for drinking and cooking, on typical municipal water.
Change any of those assumptions and the interval moves. Two people in an apartment might take twice as long to reach the rating. A household that fills water bottles for a sports team will get there in half the time. And water quality matters as much as volume: heavy chlorination consumes carbon capacity faster, and sediment blinds a cartridge regardless of how many gallons have passed.
A useful reality check: a household of four drinking and cooking with filtered water gets through roughly 400 to 600 gallons a year. Against a 100-gallon pitcher cartridge that is four to six a year. Against a 320-gallon faucet cartridge it is one or two. That single calculation explains most of the running cost difference between filters that look similarly priced — best faucet water filter works the numbers through in detail.
Carbon Is the One You Cannot Judge by Flow
This is the most important thing on the page, and it is the opposite of what intuition suggests.
Sediment filters announce themselves. They work by physically trapping particles, so as they fill up, flow drops. Pressure falls at the shower, the washing machine takes longer to fill, and eventually you deal with it. A sediment cartridge cannot fail silently.
Activated carbon fails silently by design. Carbon works by adsorption — contaminants stick to an enormous internal surface area. When that surface is saturated, the carbon stops adsorbing and the water keeps flowing through at exactly the same rate. There is no pressure signal, no visible change, nothing. The only symptom is the chlorine taste coming back, and that arrives late.
There is a second reason carbon runs on a calendar. A carbon block is a large, moist, high-surface-area environment, and one that has been in service well past its rating can support bacterial growth. That is why manufacturers specify a replacement interval rather than telling you to wait for a symptom, and why a filter that has sat wet and unused through a long vacation should be flushed thoroughly and, if it has been months, replaced.
Where a cartridge carries a health-related certification, the interval is doing real work. A lead reduction claim certified under NSF/ANSI 53 is a claim about performance up to the rated capacity, not indefinitely — which is why a certified lead filter run past its rating is not a neutral object.
The $20 Method That Ends the Guesswork
For any whole-house filter, fit a pressure gauge on the inlet side and another on the outlet side. It is the method water treatment professionals use, and it converts a guess into a reading.
- A clean cartridge shows a differential of a few psi.
- Around 10 psi the cartridge is working hard and you should have a spare in the cupboard.
- At 10 to 15 psi it is spent. Change it.
- Above 15 psi you are restricting the whole house and stressing the housing.
Write the reading and the date on a piece of tape stuck to the housing each time you change a cartridge, and within a year you will know your own true interval rather than the manufacturer's assumption. How to change a whole house water filter covers the swap itself, and what PSI home water pressure should be gives you the baseline your house should be running at.
What Shortens a Filter Life
| Condition | Effect | What to do |
|---|---|---|
| Well water with sediment | Cartridge life can fall to weeks | Add a coarse 20–25 micron pre-filter stage |
| Heavy chlorination | Carbon saturates faster | Size up the carbon stage, or accept shorter intervals |
| Hard water | Scale on RO membranes and UV sleeves | Soften ahead of the filter |
| Iron in the supply | Fouls cartridges and coats UV sleeves | Iron filter first |
| Hot water through a carbon filter | Collapses the media immediately | Always divert to unfiltered before running hot |
| Main break or street work | A season of sediment in one day | Change the cartridge afterward, not on schedule |
| Weeks of non-use with a wet cartridge | Damp media sitting idle | Flush thoroughly on return; replace if it has been months |
Reverse Osmosis: Change the Cheap Ones to Save the Expensive One
An RO system has three or four consumables on different clocks, and the relationship between them is the whole game. The sediment and carbon pre-filters cost $15 to $30 and go every six to twelve months. The membrane costs $50 to $100 and should last two to three years.
Those two facts are connected. Chlorine destroys a thin-film composite membrane, and the carbon pre-filters are the only thing standing between the two. Let a pre-filter saturate and chlorine reaches the membrane; let the sediment stage clog and the membrane is starved and fouled. Almost every RO membrane that dies early was killed by a pre-filter left in too long. Change the cheap filters on time and the expensive one survives.
A TDS meter is the honest way to monitor the membrane: note the reading on the product water when the membrane is new, and when it drifts back toward the feed water figure, the membrane is finished. How often to change reverse osmosis filters goes through the full cycle, and best water test kit covers the meters.
UV: Annually, Whatever It Looks Like
A UV lamp keeps emitting visible blue light long after its germicidal output has fallen below an effective dose. A glowing lamp tells you the ballast works. It tells you nothing about whether the lamp is still doing its job.
Replace the lamp every twelve months, and clean the quartz sleeve at the same time — a film of scale or iron on the sleeve blocks the light as effectively as a failed lamp. Put it on a calendar with a reminder, because this is the one consumable in the house that gives no feedback whatsoever. Best UV water purifier covers the pre-treatment that keeps sleeves clean in the first place.
The Short Version
Work in gallons, not months, and read the rating rather than the marketing. Sediment filters tell you when they are done by dropping your water pressure, so gauges make that precise. Carbon never tells you, so it runs on a calendar — six to twelve months for under-sink, two to three for a pitcher or faucet mount. RO pre-filters go every six to twelve months specifically to protect a membrane that should last two to three years. UV lamps go every twelve months no matter how brightly they glow. Buy cartridges in multipacks so the spare is already in the cupboard, and write the date on the housing, because the real reason filters get run long is that nobody can remember when they were last changed.
More from this hub: DIY plumbing · how to change a whole house water filter · how often to change RO filters · best under sink water filter · do water filters remove lead · how we rank.