You should cook the pork loin for at least 6 and preferably 8 hours at low heat. The low heat is important because you want the pork to cook slowly in order to get the best texture and the most juiciness. Check the pork after 6 hours and see if it is ready to fall apart upon inserting a fork. It may already be tender and ready to shred at this point. If not, let it go for two more hours; if you aren't home to check, just let it go for the full 8 hours.