Collard greens take a little longer to cook because they're tougher than, say, spinach. But they don't need the two-hour long cooking times that have been traditionally associated with them either. About 20-25 minutes in the pot are enough to make them melt-in-the-mouth tender. The best way to cook collard greens is to braise them--that is, cook them in a small amount of liquid. It's great to use vegetable stock here because it adds so much more flavor to the recipe.