Various cuts of meat take different amounts of time to cook. In an ideal world, everyone would have a thermometer — either an instant-read thermometer or a meat thermometer that stays in the roast the whole time it’s cooking.
(See Meat Temperatures 101.)
Cooking at a constant oven temperature of 300°F (160°C), a 5- to 8-lb standing rib roast will take 17-19 minutes per pound for rare, 20-22 for medium rare, 23-25 minutes for medium, and 27-30 minutes for well done.
A sirloin roast of 8- to 12-lbs will take 16-20 minutes for rare, 20-22 for medium rare, 23-25 for medium, and 26-30 for well done.
A boneless top round, by contrast, will take 28-30 minutes for rare, 30-33 for medium rare, 34-38 for medium, and 40-45 for well done.
If you roast at 325°F (160°C), subtract 2 minutes or so per pound.
If the roast is refrigerated just before going into the oven, add 2 or 3 minutes per pound.
From: How to Roast Beef (OChef)