1 For a medium rare roast: Cook the roast for 20 minutes for every pound. 2  If you were cooking a 5 pound roast, you would cook it for 100 minutes. 3  For a medium roast: Allow 22 minutes of cooking time for every pound.4  If you were cooking a 5 pound roast, you would cook the meat for 110 minutes.stimate the time it will take your roast to cook. Generally, roasting time can be estimated by how many pounds of meat you will be cooking. The length of time you cook your roast for will depend on whether you want your meat rare, medium rare, or medium.