About 20 minutes.

Red beans themselves are relatively hard in texture. It is best to soak them for about 8 hours before cooking. Put the soaked red beans in a pot and cook them for about 20 minutes before they are cooked. This is because the soaked red beans will absorb water and become soft, and the cooking time will be shortened accordingly. But if the red beans are not soaked for enough time when cooking red bean soup, the texture will still be hard.