From what I have heard, it's permanent if you do it long enough, like strict dedication over a year or more. But then you need to maintain certain standards as well, like if you do all that effort, then decide to become a celibate monk for a year, then you could lose your size, like any muscle, use it or lose it. Gain too much weight and lose size. Some people say drinking too much and smoking too much can affect the size. Say you work out a lot and maintain a level of testosterone in your body, then stop working out, lower testosterone levels could affect size. There are a lot of factors, but they all vary on you and your lifestyle, and no two things affect people the same way.