Im not an expert on SB or anything, I have only been doing it a few weeks, but I did read a Q&A on preventions website with Dr. Agatston.
Here is the question and answer.
Q: How many times can I repeat the first phase of your South Beach Diet?
Dr. Agatston: If you have a great deal of weight to lose and predominantly central obesity, you may extend South Beach Diet Phase 1 to 4 weeks, but please make sure you exercise as well to maintain your muscle and bone mass. Returning to South Beach Diet Phase 1 after being on Phase 2 or Phase 3 for a period of months is fine. I have done this myself after a vacation or other times when I have gained weight
Hope that helps you!