A cooking time of approximately an hour and a half is required for an average-sized leg of lamb cooked medium rare. Depending on weight and intended level of cooking, a bone-in leg of lamb takes between 1 hour 15 minutes, and 3 hours 15 minutes to finish cooking.
A bone-in leg of lamb is a large cut of meat; in addition, the bone inside provides some insulation to the flesh closest to it, and the cut itself is of uneven width. Therefore, extended cooking times are needed to ensure the meat is cooked thoroughly, but not so extended that the tapered end of the roast overcooks.
One method to speed up the cooking is to start the oven at 450 degrees Fahrenheit, then lower it to 325 degrees Fahrenheit after 20 minutes of cooking time. This allows for an even degree of cooking across the whole leg. Methods that call for basting also cook the roast evenly, but they slow down the cooking time as the oven must be periodically opened, causing heat loss.