It will take between an hour to an hour and fifteen minutes for a 3 ½ to 4-pound whole chicken to roast in the oven at 400 degrees F. The best way to know whether or not your chicken is cooked is by using a meat thermometer (affiliate link). A fully cooked chicken should register between 160 to 175 degrees F. when a thermometer is inserted into one of the chicken breasts or the thickest part of the thigh.