The majority of the crab that you buy at the store is pre-cooked, so in this case, all you’re really doing is heating the crab. Typically, crab legs take about 5-7 minutes to heat through. Legs tend to cook quicker than the body pieces of the crab. If you choose to cook the crab bodies along with the legs, you’ll want to add another 3-4 minutes to the cook time.