The time it takes to cook a pot roast in the oven depends on the roast's weight and cut. For example, a loin roast that weighs 2 to 5 pounds takes 20 minutes per pound to cook at 325 degrees Fahrenheit.
A crown roast that weighs 6 to 10 pounds takes 20 minutes per pound to cook in an oven set to 325 degrees Fahrenheit. A shoulder roast that weighs 3 to 6 pounds also takes 20 minutes per pound at an oven set to this temperature.
For food safety and doneness, make sure the pork roast has an internal temperature of 145 degrees Fahrenheit when it comes out of the oven.