I wash it once in 2 months or so, I have a micro sleeve from Unger, and they don't seem to get really dirty. Just give it a good scrub with your hands after you've finished and wring it out.
At the other hand, I use an abrasive pad scrubber/applicator for first cleans, and that one does get dirty real quick, the plush side is white, that is seriously stupid lol.