Convection ovens generally cook at a rate 25 percent faster than traditional ovens. Regardless of the type of oven, however, the cook time of a roast is determined by the type and weight of the cut.
A common rule of thumb when determining how long to cook a roast in a to figure approximately 20 minutes per pound in traditional oven heated to 350 degrees Fahrenheit. That means that a 6-pound roast cooks in about 2 hours. Since convection ovens distribute heat more evenly than traditional ovens, they cook food in about 75 percent of the time traditional ovens do. That means a 6-pound roast cooks in a convection oven heated to the same temperature in about 1 1/2 hours.