Slow and steady. It takes a while to put on weight and the best way to be successful in losing it is to be slow about it as well. With a well controlled diet and exercise plan, you should anticipate 1/2 - 2 lbs per week. More than that can set you up for gaining it back quickly. With this calculation, you should plan to take at least 2 months and often more for the 15 lbs. Have your physician give you a plan.