How long to boil crab for?

Not very long. Steaming is a better choice over boiling, but that can be kinda tough in a home environment. Here is the method I have used successfully with lobster for years, with a slightly shorter cooking time.

Defrost your crab legs first, of course. Gently, in the fridge overnight.

Arrange your defrosted crab legs in a heatproof container. Bring enough water to a hard rolling boil to completely cover all of your seafood. (Ensure that the water is salted, of course; as a rule of thumb it should be approximately as salty as the ocean.) After you are at a hard rolling boil, pour the water over the crab legs, cover tightly with foil (or a lid) immediately, and leave alone for seven minutes, eight if the legs are particularly thick. Remove from the water and eat.

You may optionally add other ingredients into the container with the crab: dill, fennel fronds, roughly sliced lemons (or any citrus; I love lime with crab), peppercorns, bay leaves, tarragon... the possibilities are functionally endless.

tl:dr About ten minutes, at a hard boil for defrosted crab

If you are trying to achieve a crab boil, which is sort of similar to a crawfish boil, except with whole crabs, then you should defrost the crabs (they should optimally be live when they go in but we can't have that this time, of course) using Daniel's fridge method. It is very important to cook them as close to the defrost point as possible as you don't want any more breakdown than you already have. As a matter of fact, since it's a boil you can cut it close and throw them in when still a litle frozen, say five or six hours after moving them to the fridge but no later than 24 hours after.

Once you get to the point about an hour from the time you want to eat the crabs, you want to get a pot of water on large enough to hold some new potatoes, some corn (the little ones that come presectioned are great if you don't have access to fresh corn) and most importantly, your crab. While the water is coming to a boil, you want to get your spices ready to go. I'm not going to tell you how to spice that water since that prefrence is extremely regional and normally backed up by knives and/or guns, but the two ways I normally do it are Old Bay (Baltimore style) or Tony Chachere's (Fake cajun style) along with a lot of salt and pepper. You want enough seasoning that when you dump the crab, it is thickly coated. Once your water gets to a boil, add the potatoes. Your corn should go in shortly after and once your potatoes and corn are about ten minutes from done, drop that crab in there.

Give it one good stir about half way from the end and make sure you have a place to get rid of all that boiling water. If you pot is small enough, you can just put a colinder in the sink, but if you are doing for a bunch of people the yard works great... as long as you keep people off the part where you do it until it cools down and you don't mind a bad patch of grass. Once the water is off the crabs, dump out on some newspaper if outdoors or move to a platter for the table, and serve some french bread and dig in.