How long the leg of lamb roasts in the oven depends on how well done you like it on the scale of rare to fall apart tender. Please reference below for approximate cooking times.  Keep in mind, the internal temperature of the meat will rise at least 5 degrees after it’s removed from the oven (“carryover”).  Also, depending on other factors such as lamb weight, resting time, pan used, etc., your lamb leg may roast slower or faster.  Always check early, then continue roasting as needed until the meat reaches your desired internal temperature. Rare: 125°F (about 15 minutes per pound) Medium-Rare: 130°F to 135°F (about 20 minutes per pound) Medium (NOT RECOMMENDED): 135°F to 140°F (about 25 minutes per pound) Well-Done (NOT RECOMMENDED): : 155°F to 165°F (about 30 minutes per pound) Falling-Apart Tender Lamb: About 35 minutes per pound, or until the internal temp registers over 205°F.