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I change it when the bristles are no longer flat across. I'm not sure if that's what Bubbles means by "thinning," but I notice them getting worn down in length before the bristles become significantly fewer in number. I buy a dozen a year for six bays.
We very seldom have one that we replace because they are worn out, they are usually stolen before they wear out. If one should last that long we replace them when they start to get rounded in the center.