Rebound weight gain is always the fastest, especially if you lost the original weight relatively recently. Just make sure to maintain a daily caloric surplus and you should get there in no time. Skipping meals can be an effective strategy, but you have to make up for it by eating even more at dinner than you otherwise would have been able to eat throughout the day. In the end, the most fattening diet is the one that lets you consume the most calories. Good luck!
edit: missed your question, lol. With a daily 2000 calorie surplus you could hypothetically gain as much as 200 pounds in a year. Based on what I've seen others accomplish, though, 50-75 pounds would be a more realistic estimate.
10 years
