To cook a pork loin roast safely, the cooking time is roughly 25 minutes per pound at 375 degrees for up to 4 pounds. For how long to cook a 4 lb pork loin in the oven it's about 1 hour 45 minutes, up to 2 hours. If you are cooking  a roast that's 5-10 pounds, reduce heat to 350 degrees and cook for about 10 minutes per pound.