Depending on the brand, cooking time for sliced dduk (rice cakes) will differ. Most packages instruct the rice cakes to be boiled for 2-3 minutes, but usually that is not enough time. They may be soft when pipping hot, but once they cool a little, they'll become quite chewy. Instead, cook the dduk for 6-10 minutes, or until you can easily cut one with a fork or spoon. They should be tender with a slight chew.