My dad has MG and was diagnosed in October. He is on 60mg of Pred per day, and it probably took about 5-7 weeks before we saw *significant* (50% or more) improvement, but he was also in a major Myasthenic Crisis when he was put on the Pred. He still takes 60mg per day and they have tried tappering down, but as soon as the Pred is lowered, even just 5mg, we see the change in him. Ptosis starts immediately and he has muscle weakness.