Roast the duck for about 3 hours, turning the duck every 30 minutes. 9. Remove the roasting pan from the oven and test to make sure the duck is finished cooking. 1  Insert a cooking thermometer into the thickest portion of the duck meat, either the breast or thigh.2  Be sure the thermometer is not touching any bones.oast the duck for about 3 hours, turning the duck every 30 minutes. 9. Remove the roasting pan from the oven and test to make sure the duck is finished cooking. 1  Insert a cooking thermometer into the thickest portion of the duck meat, either the breast or thigh. 2  Be sure the thermometer is not touching any bones.