How fit can you possibly get? How fast could you potentially run? How easy could you make a hike feel? Questions about our potential are always fun to ask and tricky to answer. In order to limit the plethora of valid answers I will focus on something that is easy(er) to measure: lean body mass in natural athletes.
Before we get started, there is only one point that I strongly believe in the matter:
Reaching your full potential is doable for any one. There is no such things as bad genes, just poor uses of what you've been given. If you are a 5"5, playing pro basketball is probably not a viable option. Does that mean you have shitty genes? Yea for that one situation. For all other aspects of life, you're probably good to go.
Saying things like "Oh he/she's got amazing genetics is why he/she looks like that" is (in my opinion) complete B.S. He/she has a better work ethic than you, that's all.
There are few different formulas that have emerged over the years about how to estimate the potential in lean body mass. Most of the studies have been conducted on professional natural body builders who's only goal is to maximize lean body body mass and reduce body fat. So when one of them goes on stage, in peak condition, we can get a glimpse at what is really achievable.
All the articles I've read on the matter focus on the same formulas: The Berkhan Model, the Casey Butt formula, the Macdonald Model and the Aragon model. There is another model, the fat free mass index (FFMI) but the studies don't seem to entirely reliable.
Out of these 4, Butt's and Berkhan model calculate what your peak lean weight could be while the Macdonald and Aragon model calculate how fast you can get there.
The Berkhan model
Height in cm - 100 = body weight in kilos around 5% body fat
For exemple, I am 180 cm, so 180-100=80. 80 kilos is the biggest I can get while staying super lean.
This is the most easy to use, but lacks differences in bone structure.
Casey Butt's Model
This one is more tricky to measure but is more accurate because it takes into account muscle to bone ratio. In the Berkhan model, you can have two people with the same height but with different bone density. The more bone you have, the more muscle you can support.
The formula is a little bit more complicated and since my computer science levels are as low as my squats, I'll kindly ask you to check it out the following link to figure out the answer.
The Macdonald model shows you how much lean mass you can put on after years of proper training.
These values are based off men, women can just cut those values in half.
As you can see, the longer you train, the more patient you have to be. That's why there's no point on going on crazy dirty bulks, most of the weight you are putting on isn't muscle.
Again, patience is key as you get more advanced. If you are an advanced 200 pounds man, you can look at potentially gaining 0.5-1 pounds of muscle per month.
These models give us a good idea of what our potential of lean body weight could be. There is enough research that was analyzed that we can safely assure that at the very minimum, these figures are accurate for the average lifters.
The Macdonal and Aragon models are based on each researcher's experience and provide a good general rule of thumb.
Individual differences will arise according to lifestyle, training regimen and other individual differences.
Always focus on proper training and nutrition and I guarantee you can reach your potential sooner than you think!