Pot roast takes about 8 - 9 hours on low or 4 - 5 hours on high to cook in the slow cooker. Once done, the roast and vegetables should be cooked through. u003cbr/u003eThe done temperature of a roast is 145°F for medium-rare (taken in the thickest part of the meat) and the vegetables should be tender all the way through when pierced with a fork.