About three or four times a year. Usually when the chain gets some of that dark, greasy build-up on the cassette sprockets. The build-up isn't all that much, but looks yucky to me. And, I hate fingering a dirty chain. Screws up the gloves and the white cloth bar tape. It takes a couple of hours, and an overnight to let stuff dry. I take my time and use a small, metal pan for the solvent and an old toothbrush.

p.s. For the first time in my live, I've been considering getting a tattoo. It would be on the inside of my right calf and wold be a semi-circle of little triangular shapes. Exactly like what we get when we brush our calfs against a dirty chainring. But, I survived my Navy years without getting a tattoo and at 63, I'm probably too "old" to get one now.

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p.s. Thanks all for the info. I realize that this is an emotional, and oft discussed topic. I appreciate everyone's thoughts.