Back when I had leather and crochet gloves, every month or so was fine.
Now with synthetics and some kind of foam padding, the foam starts stinking early and, once stinky, never seems to get better with washing. Scratching my nose or adjusting my glasses becomes an olfactory ordeal. So they need to be washed after every ride, sometimes with a "pre-soak" of Borax on the inside while I'm getting the rest of the laundry load together.