In many ways, how much muscle you can gain in a month, year, or any time-frame is dictated primarily by prior training experience, nutrition, exercise, and the "biological lottery" we know as genetics. Yes, your capacity to build muscle (and stay lean) is very much an inherited trait.
Now, don't misconstrue the above as saying that you won't be able to gain much muscle if your mom or dad isn't naturally jacked and shredded?
It merely means some people have a greater propensity to stay lean while building muscle mass quicker.
Unfortunately, not everybody has the genetic gifts to be a professional athlete or high-level bodybuilder. That's biology, though. You can't change the cards life deals you, but you can decide how you want to play your hand. Remember, hard work beats talent when talent doesn't work hard.
Building muscle mass is possible for virtually everyone, regardless of genetics. If you're wondering how to gain muscle fast, it's not a matter of training and eating as much as possible (unless you want to put on excess body fat in the process). Instead, it's about being consistent with your workouts, eating properly, and focusing on progressive overload.
With that in mind, let's take a more in-depth look at building muscle mass and what science has to say about how much muscle you can gain naturally.
Research indicates that specific subsets of miRNAs (noncoding micro-ribonucleic acids) differ among humans, making some of us highly hypertrophic and others lowly hypertrophic (1). It turns out that particular miRNAs are pivotal regulators of how our muscles synthesize new protein (and consequently, muscle gain).
In non-nerd terminology, this means resistance training (e.g., weight lifting) increases muscle gain to a greater degree in some and less so in others. But how much of a difference does it make in the real world?
Well, in one study, the "high responders" were able to build an average of 4.5 kg of muscle mass after 12 weeks of a push-pull-legs resistance training program (five days weekly) (2). The "low-responders" only put on an average of 1.2 kg in that same time span.
As you can see, your genetics undoubtedly play a part in how fast you can gain muscle. It appears that people with favorable muscle-building genetics can pack on lean body mass (LBM) quite a bit quicker than those with less-favorable genetics.
So, how much muscle can you gain in a month?
Chances are you'll be able to build between 0.3–1 kg of muscle in a month, assuming you lift weights diligently 4–5 times per week and consume a protein-rich diet with enough calories. The exceptions to this are inveterate trainees who are already close to their natural muscle-building limit and those who are just starting a resistance-training program for the first time.
Building muscle mass is not a linear process by any means; over time, there are considerable diminishing returns. When you pick up a barbell for the first time, your muscles are essentially hyper-sensitized to the anabolic response of resistance training (3). This phenomenon is colloquially known as "newbie gains."
You can accrue lean body mass rapidly in the initial months of your first weight-lifting routine. Heck, some people add upwards of 4–5 kg in a month during the "newbie gains" phase.
On the contrary, seasoned natural bodybuilders and gym-goers alike may only be able to gain a few grams (yes grams, not kilograms) of muscle mass in a month. And that's even if they are crushing it in the gym and eating right.
Given the extant literature, it's clear that genetic differences can account for the variable responses in muscle growth among humans that regularly pump some iron.
The more pertinent question is, can we alter our genetics in a way that allows us to gain muscle faster? In short: Yes, we can, but it's unclear how safe that would be.
Gene doping is a controversial topic, one that comes with a myriad of bioethical issues. There isn't much we can say about the health risks of transferring fortuitous muscle-hypertrophy genes to people who aren't naturally endowed with them. It's certainly intriguing to think about genetic modification as a means of bolstering muscle growth and athletic performance, but also disconcerting when you consider that it would irrevocably alter everything about society, let alone sports and bodybuilding.
If anything, we would start to take impressive physiques and freak athleticism for granted. Imposing specimens like The World's Strongest Man, Hafþor Björnsson (aka "The Mountain" from Game of Thrones), would no longer seem out of the ordinary.
It's a bummer we all can't build muscle fast and be outstanding athletes, but there's something to be said for overcoming obstacles and making the most of what you have.
Rather than asking your gym buddies, "How much muscle can you gain in a month?" you should be looking at the bigger picture. Genetics notwithstanding, muscle growth takes time. You won't pack on 5 kg overnight, not even with performance-enhancing drugs and the absolute best muscle-building genetics.
Consider that the amount of muscle you can gain in a month is insignificant compared to how much you can pack on in a year, a decade, or many decades. Your genetics play a role in how much muscle you can gain in a month (and beyond), but blaming your lack of muscle growth or being out of shape on "crappy genes" is a pretty lame excuse.
You can't always control the circumstances, but you can control how you respond. Train hard, eat right (supplement with a mass gainer if necessary), and let the chips fall where they may.
Improving your body composition and achieving your fitness goals is a contest with yourself, not anybody else. Be better than your former self. Never compare your body to someone else's; it doesn't matter what they look like — they're not you, and you're not them.
The only question you should be asking is, "What do I need to do to be better than my former self?". Frankly, that mentality will carry over to all facets of your life, not just fitness.