Just deep enough!
Sorry to be a smart ass but the depth of the biopsy
varies with the kind of skin cancer
, but rarely goes deeper than the thickness of the skin.
The area will be numbed, so, fortunately, it rarely matters to the patient how deep the biopsy extends. Biopsies of shallow lesions are usually shavings of a partial thickness of skin; deeper lesions often are punched, taking a small full-thickness piece.