It has been taking 9 months to complete the diet. In Spike's case, I have been feeding him extra. By the way, Spike is about 5 years old. I think we are working with two issues, one being the health hazard of obesity and the other is the mental well being of the pig. We must also consider the general comfort - warm, not threatened, safe and a consistent routine.
Mental health is very important.
Daily encouragement is needed. First you act like a sergeant. OK let's get up and get going. Time to eat and exercise. At the same time you give belly rubs and messages and tell them how important they are, etc.