A coat rarely touches skin directly, so it stays clean far longer than anything under it. Cleaning it more often just wears it out.
The short answer
once a season
When to wash sooner
Wash it now if
- the end of the season, before storage
- a visible stain
- the collar or cuffs have gone shiny
What to do between washes
Brush it weekly. A clothes brush lifts the surface dust that otherwise grinds into the fibres and dulls the fabric.
How to wash them
Per the label — most wool coats are dry clean, most technical shells machine wash and need reproofing afterwards.