I generally only clean my rifles three or four times a year, although I recently bought a gun vise and have been scrubbing the bores more often. With my primary hunting rifle, I don't see much difference whether the bore's covered in copper or not. I get 1.25-1.5 inch groups @ 100 yards with Express Core-Lokts, and that's plenty good enough for my shots (250 yards maximum).

My general rules of thumb are to cover every metal surface with a thin coat of oil, where safe (not the bore or chamber when shooting!), and that if Ican see copper in every groove without using a bore light, I probably need a heavy scrubbing with a good copper solvent (Bore Scrubber, Sweets 7.62 for tough cases).