The amount of time it takes to steam yams or sweet potatoes is determined largely by how you slice them prior to cooking. When cut in half lengthwise, then cut crosswise into 1/2-inch slices and placed in the bottom of a steamer basket with about 2 inches of water beneath, it should only take from 8 to 10 minutes for them to cook thoroughly. Steaming retains the flavors and valuable nutrients that could be diluted with simply boiling the yams in water.