how long should a roast be cooked for
1 How long does a cooked roast of beef last at room temperature? 2  Bacteria grow rapidly at temperatures between 40 Â°F and 140 Â°F; cooked roast beef should be discarded if left for more than 2 hours at room temperature.
After that, you're going to want to cook it 20-30 minutes per pound (if it's a butt roast, you might bump that up to ~40 minutes). I'd start checking it with the thermometer at the very bottom end of that range, and take it out of the oven before it hits your target temperature.