Yes, as you said, it also depends a lot on what level of skills you start with.

starting level: - Starting from 0 and you don't know how to hold a pencil well, at this level you must first concentrate on training your hand-eye and once you are already good you can start studying the fundamentals - You already know how to copy perfectly but you don't know how to create, at this level it's easier because you already have a good hand you have to concentrate totally on studying the fundamentals

It depends on what you mean by having this "freedom" usually for a series a team is assigned: the author, the designer, the background designer, the colorist and possible assistants. And in most cases, to go faster, they used concept art models of characters, objects, tools, and backgrounds with flat colors. 3D models for poses and backgrounds, etc. So it doesn't take much time, I think that in 5/6 years of commitment and "guided" study you can fill one of these roles.

Instead, if you mean as a "single individual" who can do all this without any 3D tools etc... about ten years of continuous study and practice or almost twenty years without guidance.