On my stove, this recipe cooks for 20 minutes while set at Medium-High. However, as several readers have confirmed, that is not true for all stoves. Lower the cooking temperature as low as possible, while still maintaining a constant boil. When the jam stops foaming, test the jam by dropping a teaspoon's worth onto a plate. When it thickens and doesn't run, it is done. This may take as little as 10 minutes, or as long as 20 minutes, depending on your stove.