Since five or six. Doraemon inspired me, I have been one to create spinoffs from series I like that include me (like if in Doraemon, somehow I am in the gang lol).
I learn by myself, especially my current style. I do not major, nor actually ever going into informal classes for drawing. Manga and internet's been my best buddy for this.
First, I learn from doraemon. Then, I stumbled upon shoujo manga and started to draw in shoujo manga style. Then, I stumbled more references via social medias (not that manga, but kinda manga? Semi realistic, idk?) Then somehow mine developed into my current style.
How long is a hard question. I think we learn until we stop. But if you ask my art progression, it's my whole life progressing until stopped when I got into accident and gotta redo the progress for 3 years.
How? Practice. See references. See other artists' creations. Can't say I am good in anatomy and sometimes misses some parts. I am not familiar with rendering as well. Still learning color theory. See others' videos and tips, try to apply, stick if it fits.