People ask me how long it took Lincoln to get to his current size. I'd have to say over a year and a half. He just started gradually gaining weight until we realized that he had a serious weight problem.
Within a year of having both cats we realized that we needed to try to do something to get his weight down. Several unsuccessful attempts later, I begged our vet to prescribe food for him. He was pretty unresponsive about it, and said that we needed to accept his weight the way it was. Finally, when I took Belden in for a routine check-up, I mentioned to the vet (different vet, same clinic) about Lincoln and his weight problem. He said he could prescribe the special diet food for Belden since she was a little chunky (not too bad, her weight has stayed the same) and we could get it for both of them. Within a month of putting both cats on the diet food we noticed Lincoln's weight went down to 30 from a high of 34 pounds.
Here are a couple pictures to illustrate how he looked:
Lincoln and Belden in April 2002 (15 lbs.):
(Belden is gray, Lincoln is buff colored)
Another shot:
Lincoln in February 2005 (30 lbs.):
Another shot: