1. Training Age
Your training age is how long you’ve been lifting weights. If you’re a newbie, you will be able to gain more muscle faster than if you’ve been lifting for many years.
2. Hormonal Profile
The amount of testosterone and other muscle building hormones in your body can vary because (1) your lifting regimen does not help elicit a testosterone response (i.e. not lifting legs, or not lifting heavy enough), or (2) your body simply doesn’t produce as much testosterone…which brings me to the next variable.
There is a concept of a genetic bell curve, which I discussed in the maximum muscle potential article. To summarize, some men are naturally inclined to build a lot of muscle because of factors such as hormonal balance, or the thickness of their frames whereas others have trouble building much muscle no matter how hard they try. Most people of course by definition (roughly 68%) are genetically average.
4. Muscle Memory
If you are 180lb then you decided to train for a marathon, you may lose a solid 20lb of muscle. So how long will it take you to gain back those 20lb of muscle back? Answer: Not long at all. Maybe only a 1-2 months, because your body has a mechanism for restoring the previous homeostasis, which is often referred to as “muscle memory”.
Heavy duty supplements like steroids, or other potentially harmful, performance enhancing substances that can help you build muscle much faster, but I’m assuming you are far too smart to do that! Other less dangerous, but still potentially harmful substances like Creatine can aid in muscle growth as well.