After every wear. They either smell of urine from mucking out or are covered in dust from filling haynets or hair from riding bareback or grooming or him rubbing his head on me or they have saddle soap smeared on them.
Or lately every item of clothing I wear gets chewed as there is a new foal on the yard and next youngest horse (3yr) gets jealous and has started grabbing clothes and stealing scrunchies again. Horses are terrible for your wardrobe.