The short answer
For most homes in the Netherlands: every eight weeks. In a city centre or on a busy through road, every four. If your house sits quietly and in the open, three or four times a year is plenty — and anyone selling you more often than your situation calls for is selling you a subscription, not clean windows.
What dirties glass is almost never "dust" in general. It is three things, each with its own rhythm: exhaust and brake dust from traffic, pollen in spring, and salt and grit deposits carried in by rain and wind. A house on a main road takes as much of the first in a week as a house in a courtyard takes in a month. So the question is not "how often" but "where does the house stand".
There is one practical threshold that surprises most people: deposit that has dried and been wetted again bonds to the glass. Clean before that has happened and the work is quick, and therefore cheap. Wait until the glass looks visibly dull and it has to be soaked off, and the same visit takes longer. That is the real reason a regular interval costs less per visit — not the discount, but the fact that there is less work in it.
Interiors are a different story. They get dirty from cooking, candles, smoking and pets, and that happens far more slowly than the outside. Twice a year inside against six times outside is the right ratio for most households. Anyone having the inside done as often as the outside is paying for something that was not dirty.
And the honest part: if you have a detached house out in the country, with little traffic and no trees against it, four times a year is plenty and eight is money thrown away. We would rather say that now than have you work it out yourself after a year.
What to remember
- Eight weeks is the right rhythm for most Dutch homes.
- Position decides the answer, not the type of house or the number of windows.
- Interiors need doing a third as often as exteriors — do not pay for more.