How long does it take for the scar of an adult male circumcision to disappear?

A week. Circumcision if done correctly leaves no scars. The scab usually takes about a week to form and fall off.
Few weeks. Scar never disappears, but becomes less and less noticeable as time goes by. At least a month before it is not noticeable in most cases. There is almost never a scab present because the skin edges are either sewn or glued together.