I think the time required to steam crabs varies depending on the size of the crab and the intensity of the heat. Generally, medium sized crabs need to be steamed for 15 to 20 minutes while large crabs need to be steamed for 20 to 25 minutes. If you use high heat, it will take less time, while low heat will take longer.

Before steaming the crabs, you can soak them in cold water for some time to make them spit out the dirt and toxins in their bodies. In addition, you can add appropriate amount of ginger slices and scallions in the steamer to increase the flavor of the crabs and remove their fishy smell.