Why was it on? Did you break an ankle? Have surgery? Sprain your ankle? More info is needed. Keep in mind, your foot wasn't moving once you start flexing and extending your foot the swelling should go away faster. Warning: one fo the complications of a cast could be a clot in the leg which sometimes people notice as swelling. So if you have any pain or swelling in the calf inform a dr. Asap.
Part of . The answer will depend on why you were wearing the cast. Was it after an ankle fracture? Surgery? Or just a bad sprain? The more trauma likely the more swelling. As you start to walk and place the foot through range of motion, elevate the foot, offer compression , the swelling will begin to reduce.
After therapy. Should start to go down after you start range of motion and stretching exercises to help the muscles pump out the excess fluid.