Every other day to avoid overtraining. Mixing it up prevents plateauing. You can also vary sets, use pyramid sets, thicken the bar, add weight in say a backpack, or even try one-arm emphasizing versions.

Biceps, contrary to popular belief, are necessary in boxing. While they might not do nearly as much direct work as do the triceps, working them prevents muscular imbalance which, let me tell you, is not a good thing. But please don't go bananas over them like the bodybuilding world