Porch overhangs can depend on your intentions for your porch, but they are usually at least 12 inches away from all sides of the roof.
Extended overhangs that reach up to two feet are typical for people who want more shade or protection from the elements.
You may even choose to have an overhang shorter than 12 inches if your area is regularly dry. Choose a length appropriate for your needs and keep it uniform across the roof's edge.