Once a year for a normal household. Five things push that shorter, and if any two of them apply to you, every six to nine months is closer to right.
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A dryer sheds lint every load, and the trap catches only part of it. The rest settles in the duct, slowly, in the places where air slows down. A year of normal use is roughly how long that takes to go from harmless to measurable in a typical house with a short run.
The number is not the point. The interval is really a stand-in for two questions: how much lint is going in, and how easily does it get back out. Everything below is one of those two.
How many loads you run. A household doing eight or ten loads a week puts through two or three times what a couple doing three loads does. That difference alone can halve the interval.
A pet that sheds. Hair comes off clothing and bedding in quantity, and it binds lint together rather than passing through as loose fluff. It is the most underestimated item on this list.
A long run. The further the air travels, the more chances lint has to settle. A run across a house behaves nothing like one where the dryer sits against an exterior wall.
Bends, and especially vertical ones. Every turn is a place where air slows and lint drops. A vertical run adds gravity working against you the whole way. An upstairs laundry room usually has both.
A roof cap instead of a wall hood. Caps are higher, longer to reach, and more likely to have a flap that has stopped closing properly, which is how a nest gets started.
Ground-floor laundry room, short run to an outside wall, two people, a few loads a week: once a year, and you will probably get away with longer.
Upstairs laundry room, roof vent, family of four, dog: every six to nine months. Two of the five above stack, and this is the case where people are surprised by how much comes out.
A dryer running most days of the week, whatever the run looks like: every six months. Volume beats geometry.
These are intervals to plan around, not a schedule to obey. The dryer tells you the truth, which is the next section.
If a normal load starts needing two cycles, the vent needs clearing regardless of when it was last done. Drying time is a direct readout of how much air is getting out of the house, and it moves before anything else does.
The same goes the other way. A vent cleaned eleven months ago on a house with a short run and a dryer that still finishes a load in forty minutes is not urgent. The full list of signs is here, sorted by how much to trust each one.
Clear the lint trap every load, and once in a while pull it out and vacuum the slot it sits in. Lint gets past the edges of the trap and collects down there.
Once or twice a year, pull the dryer out and look at the flexible hose behind it. Crushed flat against the wall is the most common fault in the whole system and it costs nothing to fix.
Check the outside hood while the dryer is running. Warm air should push out steadily and the flap should open. A flap that never opens is a blockage or an invitation to a bird, and often both.
A new house is not a clean duct. Ducts sit open on a job site, and drywall dust, sawdust and offcuts end up inside them, so the first cleaning on a new build often pulls out more than a five-year-old house does.
A new dryer does not reset anything either. It is connected to the same duct that slowed the old one down, which is how a lot of vents get their first cleaning shortly after somebody replaces a machine that was never the problem.
Usually not. Volume is the strongest single factor, and five people is roughly double a typical household. Every six to nine months fits better.
Stretch it. A short run and a few loads a week can go well past a year. Watch the drying time and the outside hood rather than the date.
Often more often, because the run is usually longer than it looks and frequently shared. Whole buildings are worth doing on a schedule.
No. They change how the clothes tumble or how the hose attaches. Neither moves lint out of a duct.
No, they are a different job on a different interval, and most homes do not need them touched nearly as often. The air duct page explains when it is worth doing.
If nobody can tell you, treat it as overdue. Houses change hands with a duct nobody has ever opened, and that is the common case, not the rare one.
Tell us the run, the household and what the dryer is doing, and you get a straight answer about whether it needs doing yet.