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.