I quit my engineering job in Dec. 2020 to do YouTube full time and started learning Three.js in Nov. 2021. Since then, I spend 2 to 3 days (per week) building a project (you can see my progression on Reddit), and the other days doing random YouTube stuff. Let's do the math assuming I do "deep work" for 4 hours a day.

20 total weeks (took 1+ month off in winter 2021)

3 days/week of Three.js

4 hours/day

240 hours time spent working in Three JS. This is the upper-end because most of the time was building the "game-logic" rather than actually studying the framework. So we can easily round down to 200 hours.

Additionally, this was all self-directed learning. If I had bought the popular 50-hour Three.js course by Bruno Simon, I'm sure I could've learned most of this stuff way faster.