What to know
Why nobody can quote you a duration
Cleaning removes what has accumulated. It does not change what the house produces, which is what put the material there in the first place.
So the honest answer to how long it lasts is: as long as it takes your particular household to put that much back. That is a different number in every house, and anybody quoting you a fixed span is guessing.
What shortens it
- Pets, particularly several, and particularly ones that shed.
- Carpet throughout rather than hard flooring.
- A busy household with a lot of coming and going.
- Any building or sanding work, which can undo a clean in a single week.
- A filter that is the wrong size for its slot, so air bypasses around the edges.
- Long filter intervals, which is the most controllable factor on this list.
What lengthens it
- A correctly sized filter, changed on condition rather than on the calendar.
- Sound duct connections, so unfiltered air is not being drawn in on the return side.
- Hard flooring and fewer soft furnishings.
- Keeping registers open so the system is not running at raised pressure.
What to expect afterwards
The visible parts — register faces, the first stretch of the boot, the return grille — will look clean and stay looking clean for a while. That is the part you can check, and it is a reasonable proxy for the rest.
What will not change is dust settling on your furniture, because that is generated in the house rather than delivered by the ducts. Being clear about that up front is the difference between a customer who is satisfied and one who feels sold to.
How to know when it is due again
Check rather than count. Pull the main return grille once a year and look. If it looks close to how it did after the clean, you are fine.
When it starts looking like it did before, that is your interval — and now you know it for your house rather than for houses in general.