Cooking time for lentils can vary from 15 minutes to as long as 1 hour, depending on the type and age of the lentils. Red Lentils (which turn yellow when cooked) cook very quickly because they are split and after 15 to 20 minutes they soften to a puree. Brown lentils hold their shape better but can take longer to cook. Serve this soupy stew-like dish in a shallow bowl over brown rice. I got this from Prevention Magazine.