AWS D1.1:2004, 5.15 simply says that "Surfaces on which weld metal is to be deposited shall be.....Surfaces to be welded, and surfaces adjacent to a weld shall be...."
I don't see a definition of "adjacent" that describes it in a way as to give a specific dimension to clean on either side of the joint centerline.
In other words I guess you use your best judgement in your cleaning to achieve acceptable results.
John Wright