The Betty Crocker website contains a chart explaining the cooking time for pork loin roast as well as a number of other cuts of pork, beef, veal and lamb. The chart also includes the cooking temperature, the desired internal temperature and instructions for letting the meat stand before serving.
For a pork loin roast weighing between 2 and 5 pounds, the recommended roasting time is 20 minutes per pound at 325 degrees Fahrenheit. Using a meat thermometer is necessary to ensure that the interior of the roast reaches 145 degrees Fahrenheit. After roasting, the roast should be removed from the oven and allowed to stand for a minimum of three minutes before slicing and serving.