I use a microfibre towel, and basically, I was it whenever it looks like it needs it. I usually fold it in half and the half again, so when the four quarters have been used so that you can see a discolouration (front and back) then I wash it. There's no hard and fast rule for washing, as it depends on how much oil you're bowling on. I think I probably clean mine once a month or so, which works out at about once every 50 games, or 6 games per "side" of towel.
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Reporting from England