I started taking drawing seriously in 6th grade, and while I had a gradual if slow improvement over that time, I had a definite jump in quality just as I left high school and headed off to college, which is about when I joined DeviantArt and hooked up with a bunch of other, way more serious artists whom I looked up to and admired. Then I had a period of rapid improvement, followed by a sudden sharp artistic depression as many of the artists I looked up to became hateful and self-destructive, so I ended up withdrawing for a bit and plateauing until I was out of college and willing to pick things back up again and take it at a much more relaxed and casual pace.
I haven't made a tremendous improvement since then (and sometimes I look at my older work and think I was BETTER before my depression), but I've definitely branched out and am way more flexible with my art now, along with just generally having a better grasp of certain fundamentals that I shunned before.
To break it down, I've been drawing seriously for about 17 years (though perhaps more like 15 by discounting the periods of depression), and it took me about 7 of those years in the beginning to actually make a marked improvement.
In other words,
art improvement takes time and doesn't happen overnight! One might have an epiphany down the road that causes a sudden jump in quality, but a foundation needs to be established
first before such a revelation can occur!
I'll leave you with this illustration (which I shared during one of Szalia's streams but is pretty appropriate here).