Cooking time depends on the size of the cut, as well as how you like it cooked. The perfect corned beef is tender enough to slice, but without totally “chipping” and falling apart (falling apart is terrific for corned beef hash, but if you’re having guests, slices that hold together are a bit more appetizing). Test the meat with a fork, or cut a small piece and taste for tenderness. When the fork goes in without much resistance, you’re about there.