I dont wash until its visibly/physically dirty, or if it starts to get stiff. I usually wash about every 2-3 washes though.....Im kind of hard on them...I will lean them over my truck bed rail to dry when im finished drying.....
I pre-dry with another MF then finish off with the WW and I get 12 or so dryings. But then I am only using it on my car and no wheels , door edges etc.
I think if the car is washed very well and rinsed .. not a lot of dirt for the WW to pick up.
I wash mine after every use, though I might use it on more than one vehicle if I'm doing them at the same time. Between picking up dirt from :nixweiss and absorbing minerals/etc. from the water, I'd rather be safe than sorry.
Heh heh, there's an old joke about a guy who never washed his towel since he only used it when he was fresh from the shower....he finally asks somebody "are towels supposed to bend"?