Aside from the cooking time listed if you are following a specific recipe, color is the primary indicator when your roux is ready. A white roux will only take a couple of minutes to cook, and will be very light in color. A medium roux will typically take 10-15 minutes to cook and will reach a color almost of peanut butter. Dark roux will take up to half an hour to cook, and will reach the color of milk or dark chocolate, depending on how dark your recipe calls for. Roux will also begin to develop a rich, nutty aroma as it reaches the medium coloration, which will grow stronger as it darkens.