Less often than you think, and that's not an excuse to be careless — it's just how wool actually behaves.
The short answer
Wool resists odour on its own, so a pair of merino or alpaca socks can usually go two or three wears between washes if you're not doing anything strenuous in them. That's not true of cotton or synthetic blends, which pick up bacteria and start to smell after a single wear.
Why this actually works
Wool fibres have a natural, waxy outer layer that resists the bacteria responsible for odour, and the fibre structure absorbs moisture into its core rather than leaving it sitting on the surface where bacteria thrive. Overwashing doesn't make the socks any cleaner in a way you'd notice — it just wears the fibres down faster than normal use would.
When to actually wash them
After anything sweaty — a long walk, a workout, a hot day in boots. Otherwise, airing a pair out overnight between wears does most of the work washing would.
How to wash them properly, when you do
Cold or cool wash, wool cycle if your machine has one, and skip the tumble dryer — heat is what actually damages wool and alpaca fibres, not the washing itself. Lay flat to dry rather than hanging, which can stretch the shape out of the heel and cuff over time.
A wool detergent such as our Knitwear & Delicates Detergent is gentler on the fibre than ordinary powder, and a washing net bag keeps pairs together in the drum.
Full detail on this lives on our care guide, including what to do about pilling and how to store them between seasons.
