There are two reasons why stiffness seems to last a long time after casting a joint for a fracture. First there's the joint itself. After injury, the repair processes can be damaged and slow to return to normal.
At the same time, animal studies have shown us that muscles shorten up when joints are immobilized. The tendons shorten up and the cartilage in and around the joint becomes unable to slide and glide.
Exercise seems to be the best solution to this problem. Studies haven't shown that one type of exercise is better than another. In fact, a recent study from Australia showed that patients who exercised without stretching did just as well as those who exercised with stretching.
Give yourself at least four to six weeks to regain your full motion. Check with your doctor if you have any questions or concerns.