I've had my 204 for about a month and I am very impressed with it's ease of use and the fine results I have been able to achieve.
I find that I tend to clean the stones with the usual Scotch-Brite and Comet materials after every 3 or 4 sharpenings. I might be overdoing a bit, but I feel that I can't hurt the stones and I feel more comfortable knowing that I will be able to get the best edge on my blades. Clean stones will simply cut better.
It depends on the size and sharpness of what I am sharpening. When I'm touching up my Delica Ltwt., the stones don't need cleaning until after several times, but when I was sharpening a dull sword with a 30" blade, I had to clean my sharpmaker quite often because the steel kept building up on it.
I've noticed that in the end it saves time to clean them pretty often. I think that, on the white stones anyway, there's a big difference in how the stones cut after just one sharpening (or maybe two). I try to clean them fairly often (every other sharpening or so, at least)
