I always thought he did it in less time, at least according to him:
"I stuffed myself after The Machinist. My first screen test for Batman Begins was a month and a half away after finishing The Machinist, so I had to get my weight back up. Then I had about five months to reach what you see in Batman Begins. At the start I was eating often and I was eating foods very high in calories because I lost 60lb."
Christian Bale has become the master of shape shifting. Here’s how the Christian Bale Transformation took place for his most famous roles in his own words
www.trainmag.com