how long to cook a pork roast in the slow cooker - for pork loin, 4 to 5 hours on low for tender, juicy perfection. pork shoulder needs to be cooked for 10 hours on low for a 1.5 - 2.5kg / 3 - 5 lb pork roast. shoulder is a tougher cut of pork so it requires a longer cook time to break down the fibres and fat.