In answer to how often should you clay, it's down to usage more than anything. But a good rule of thumb, as you can't feel all the bonded contaminants with your fingertips, put your hand inside a very, very filmy, clear plastic bag, and run your finger tips over the paint. You'll then be able to feel where all the contaminants are bonded to the paint.
I use a very clever type of bar, not a clay bar as such. Much better for the paint.
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