How often do people have this done?

It varies a lot from house to house and from service to service. There is no fixed schedule, so we would rather tell you what we see when we come round.

A house on a busy road gets dirty faster from dust and soot. Trees nearby bring leaves, sap and pollen, and fill the gutters faster.

A facade or roof that faces north or is in the shade dries slowly after rain. Moss and green algae grow faster there.