Boiled crab legs are an easy and fast idea for lunch or dinner. If you are going to cook frozen crab claws, it will be enough just put it in boiling water for a couple of minutes. Cook the medium size crab for 10-15 minutes after boiling, the large size crab for 30-40 minutes. Cook the legs of crab for 4-5 minutes or until red. Don't overcook it!!!