From an infection point of view I don't think there is a real need to wash a mask ever if you have the opportunity to let it sit for a couple of days between wearings. I don't actually wash any of my masks very often because I have so many and I rarely wear any of them for longer than an hour at a time. But sometimes they just need washing for sanitary reasons, like any scarf or similar item worn next to the skin. I don't think it's something to stress about, just something you do when it seems like a good idea. After I wash them I carefully fold and smooth them so the pleats lie nicely and set them out to dry on a towel. Sometimes I iron them just to make them look new again. I don't think there is any particular reason to do any of that, I just enjoy fussing with them and making them look pretty.