Well, generally speaking, no matter what kind crab you are cooking (i.e. King crab, Dungeness crab, Blue crab etc.) there are certain little "rules-of-thumb" one can follow. For instance; are you boiling fresh, raw crab? if so then I would suggest boiling them for a good solid twenty minutes in your favorite crab-boil spices. ( also the crab will change color as they cook....generally from a brownish or blueish color to a reddish orange). Are you boiling pre-cooked, frozen crab or perhaps just some crab legs?......Then you only need to re-boil whole frozen crab for 10-15 minutes to fully heat it up. If doing some good crab legs....then all one really needs to do is steam them until hot and ready to serve. Enjoy!