It's up to you & your doctor, but typically, the longest single round of hCG is 40 days. After that, you are supposed to take a 3 to 6 week break (the length is decided by you & your doctor) to prevent your body from developing an immunity to hCG (which would render it useless to you), and so that you can do a maintenance phase to stabilize your weight loss (meaning, to ensure that it lasts). I'm currently on my 5th day of Releana (hCG) and my 2nd day of the 500 calorie diet. I did A TON of research before taking hCG, because I'm a skeptic by nature. It is really, extremely helpful to be going into this armed with information. I would really suggest you do your homework, and also read the book "Pounds and Inches" which the entire hCG phenomenon is based on. You can find a PDF of the whole book online. The more you know the better you will do. That said, I recommend the program; I've already lost 3lbs! I feel a little hungry, but it's very manageable, and I expect it to subside when my body fully adjusts, anyway. From everything I've read, including clinical studies, I really do believe it's safe & effective, & targets specific fats while protecting muscle. And that makes sense anyway, because hCG naturally occurs to enable a mother's body to feed its fetus in the event of a nutritional depletion. Stored fat, not muscle, is what the baby would need, and pulling from stored fat, not muscle, is what would keep the mother's body healthy enough to carry a pregnancy to term. So, I mean, the science of it DOES make sense in accordance with the multitude of proof across virtually every species ever that nature is always in the business of promoting survival. So, like I said, I recommend the program, but I also recommend educating yourself on it as much as you can.
I have heard from someone that was on it that you have to wait about 2 weeks in between but I don't know why, by the way, my friend has tried every other diet on the market practically and the HCG injection diet is the only one that has actually helped her lose weight, she has been on it for about 2 weeks and already lost 15 lbs.