I am somewhat new to the 3D world as I've only been doing 3D design work for about 2 years. I've learnt a lot in those 2 years and am soon off to university to study more in 3D design and animation.
I don't know if I have an answer to how long it would take to earn a living from being a 3D designer but from this website and also small jobs I do from people I meet on this site, I earn enough to go out and do things I wanna do. In terms of earning a living its for sure up to a 6 year commitment depending on how easy you pick it up. I've been quite lucky and been able to have access to programs others can maybe afford such as Substance Painter.
Regardless how long it takes to earn a living or if I even will, I plan on modelling for as long as I can as I just love doing it.