Lamb cooking times vary by cut of meat, weight and desired doneness. A whole leg of lamb that weighs between 5 to 7 pounds takes 20 to 25 minutes in a 325-degree Fahrenheit oven for medium doneness. A 7 to 9 pound leg takes 25 to 30 minutes per pound.
A rib roast or rack of lamb that weighs 1.5 to 2.5 pounds takes 35 minutes per pound for medium doneness in a 375-degrees Fahrenheit oven. Lamb chops on the grill take eight minutes per side on high heat for medium doneness, while 1/2-inch patties take three minutes per side to cook on the grill.