I first started lifting halfway through my senior year of high school and went consistently till I was about 21. I'd say I made newb gains then. I went from 140 to 167 over about a three year time span. But, I ate like garbage, drank a lot and didn't really have a good program. So I made newb gains but it wasn't earth shattering.
What you're referring to is max genetic potential. You can look online and find calculators that help you determine what that is based off of a few factors... height, ankle and wrist measurements, percent body fat etc. Of course it isnt' going to be 100 percent accurate because there are so many factors that affect it.
I stopped lifting for nine years until I turned 30 and realized I was starting to get out of shape and needed to get back to the gym. I actually hired a boxing trainer and started competing about a year later. I also started a modest weight training program during that time and I quickly got back what muscle gains I made all those years ago. But, like before.... I wasn't on a tight program and was still not eating right.
When I quit boxing a few years ago I continued lifting and have been slowing adjusting my program. Now, over the past year or so I've been working on progressive overload and fine tuning my diet. It's made more of a difference in this past year than it had for the years before on a shitty program.
Point is, you hear about maximum genetic potential all the time but it's hard to even get there if you aren't training right. In a perfect world someone might be able to get there in three years. But chances are you're not going to and may never. Yes, you'll gain quickly in the first year, at about half the rate in the second and maybe a quarter of that in the third. Every year after you might only put on two to three pounds of muscle every year after. But again, that's only if you're dialed in. Don't need roids to hit your potential, you just need hard work and dedication. PEDs put you over what's natural and if looking like a freak is your thing, more power to you.