Wow I love this stuff! LOL
I wash them every time I use them unless they are my designated interior dusting towels, which don't get washed quite as much.
Any time a towel touches something that isn't perfectly clean it gets washed . . . I keep different colored towels in different ziplock bags (100% dry) for different uses i.e. I don't buff out KSG with towels that have been used to QD a car that isn't clean.
Any towel that gets dropped is washed very thoroughly and inspected, then washed a couple more times and not used for buffing or rubbing any more.
Did I mention that I like microfiber?