Some cooks cook their mixed greens (collard, mustard and turnip greens) for a long time (like hours),  just like their Mother and Grandmother did. The actual greens themselves don't take that long to cook, it's the broth or pot liquor that takes the most time to develop. Cook the protein and seasonings for one hour. Add greens and cook on low for about 1 hour, until the greens are tender and the pot liquor is flavorful.