It is almost impossible to overcook a lamb shoulder, and it is ready when the meat falls apart when touched with a fork. The cooking time depends on the size of the lamb shoulder. For a shoulder joint with bone in: 1 kg -1.5 kg (2.2 lb - 3.4 lb), you need to cook it for 3 hours - 3 hours 30 minutes. 1.6 kg - 1.9 kg (3.5 lb - 4.1 lb), cook it for 3 hours 30 minutes - 4 hours. 2 kg - 2.5 kg (4.4 lb - 5.5 lb), cook it for 4 hours 30 minutes - 5 hours.