More or less 15 years ago I talked with a guy who used to live and train in Brazil and he told me the story of how one time a group of guys from a small European country came to train where he trained (Carlsons Gracie Academy), and they had a really good level, at that time it was comparable to a Brown Belt.
When they were asked who was his trainer, they told them that they used instructional videos, studied them and trained and rolled with eachother.
Also, Pat Miletich did the same…put a Gracie VHS tape a mat in front of the TV and drilled the technique until he grasped the basics of it. Bas Rutten did the same also
Patt mikitech in highschool was in a wreslting ream with mark kerr. He also wrestled in juco in iowa. Not your avg guy.
I get it, its easy to bring up supposed "some guys from a mysterious land in europe saw a tape and became brown belts" . Most likey they had some kind of grappling base (most likey judo) and went to seminars around Europe. Maybe they were brown belts who knows. Even if they were they are a miniscule portion of the population.
Your avg guy probably needs 4~6 years of organized classes. Be about high blue/low purple to advance on his own correctly. I saw self traind guys they advanced (as beginners) much slower than they would have at a school.
So now we have to add things like "join open mats", maybe take a private, have some advanced guys train with them once in a while, have a guy maybe show him stuff every once a while. Ect ect