Find out how to cook a whole chicken here. Knowing how long to roast a chicken is key -- here are the calculated exact cooking times by weight. 2-1/2 to 3 pounds: Roast at 375 degrees F for 1 to 1-1/4 hours. 3 to 3-1/2 pounds: Roast at 375 degrees F for 1-1/4 to 1-1/2 hours.