Per our usual nagging, you should cook it until an instant-read thermometer inserted into the thickest part of the roast measures 160°F (70°C). The United States Department of Agriculture says that will produce a medium roast, the least well-done it will allow. Some people like their pork roasts less well done, but the USDA won't have it.

In a 325°F (160°C) oven, it will take between 35 and 40 minutes per pound to reach a center temperature of 160°. So you could check it after 2-1/2 hours, but it may take as long as 3 hours. The reason you really need an instant-read thermometer is that some ovens are inaccurately calibrated, some people let the roast come to room temperature before they start cooking while others put it in the oven cold from the refrigerator, and the USDA wants you to have one (we do too).