Honestly, most people get three to five wears out of a merino shirt before it needs a proper wash — sometimes more if you're airing it out between wears and not sweating through a full day in it. That's not a rounded-up marketing number, it's just how the fibre behaves.
The reason is straightforward: wool's surface resists the bacteria that cause body odour, rather than absorbing sweat and holding onto it the way cotton does. So the shirt doesn't start smelling the way a cotton one would after a single wear — it genuinely doesn't need washing yet.
The honest caveat: this isn't a free pass to never wash it, and it's not the same for every day. A hard, sweaty day — a long flight, a workout, a hot kitchen — is going to use up more of that runway than a normal day at a desk. Use your nose, not a fixed number. Hang the shirt up to air between wears rather than balling it in a drawer or a suitcase, and you'll get the most out of it. Wool actually prefers rest over washing — the fibre relaxes and any faint odour lifts out on its own overnight, which is most of the reason it lasts as long as it does between proper washes.
It's also why I could wear the same Bruce for four or five days on a trip and not think twice about it — I did exactly that across Italy and Ireland this July, no washing machine in sight for two weeks straight.
— Jock
