In my youth, when my knees still allowed any kind of pressure upon them, I'd own and wear a single pair of black denim trousers pretty much every day for as long as possible, and despaired when they had to go in the wash and I'd need to wear something else... Then I got a second, identical pair, and wore those every day all day until I had to wash them and wore the old pair and felt sad about it instead.

These days, I'll wear the same outer skirt for up to a month if nothing happens to it, and just change the layers under it whenever necessary. Saves water and effort!