A standard 4.5-pound pot roast typically takes four to five hours on the highest heat setting to roast in a crock pot. Changing the setting to the lowest heat increases the cooking time to eight to 12 hours. Since each crock pot is different, consult the owner's manual for exact cooking times and temperatures.
All types of beef, including roasts, are more tender when cooked at low temperatures in a crock pot. Add vegetables like onions or carrots to the pot along with the roast, and cook for the same amount of time. Add ingredients like potatoes, rice or noodles in the last two hours of cooking.