I always put a leg of lamb on at 4pm in the afternoon to be ready by 7pm. I cook it on 200 deg for the first hour and then 160 for the rest. When you think it is close the bieng ready, put a skewer into the meatiest part of the leg and if the skewer is hot when you pull it out and the juice that comes out is clear, then it is ready.

ETA - if the leg you have is long and skinny looking, then it may only need 2.5hrs, but if the leg is shorter, but thicker, then it will need the full 3.