Once a year whether it needs it or not.  Admittedly, I keep 2 and usually wind up switching about half season, so if you only use one, maybe wash at beginning of season, and at the halfway point.

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9~

Ain't Skeered!

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

As you can see there's no "one rule fits all' here , just keep it clean ! Don't take this wrong, but ask yourself, how often do I/wife/girlfriend wash the bath towels or dish cloth ? Same basic concept, used to get something clean.

Me, I wash mine every 3-4 days when I wash the towels I use in the shop, but that's just because I have a handful to wash anyway.
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az guy aka: R & L Bowlers Pro
rlbowlerspro@cox.net
www.rlbowlerspro.com