@Adam-Thornton Hi mate, I essentially started taking illustration seriously about two years ago, before that i would pick up a pencil about three times a year. I would say from my experience, you definately need a loose framework for study, but dont be too rigid or unrealistic with your goals. Its a massive undertaking, and you'll be studying for the rest of your life no matter how good you get, so theres no rush.
Dont say things like "i want to master drawing heads" and then spend six months drawing thousands of heads, you'll just end up giving up altogether bored out your mind, and theres too much ground to cover with everything else. Try and learn the basics, the fundamentals, perspective, value, composition etc, just allocate maybe a month each, then move on. Then you can return to each for a more in depth study of the details, the contour of each individual finger etc. You'll have to revisit these things again multiple times anyway, you cant just learn things like human musculoskeletal anatomy in a couple of months and retain that knowledge for the rest of your life. Colour theory is so complex too, for me im constantly reading about how light interacts with different materials, and the more you understand a topic, the deeper you can take your study. Its a craft, theres no end to it, its just a question of how good you want to get.
When you first start out, youre very motivated, but progress in art is so slow, its easy to become demotivated quickly. I got so fed up, i found my study and just ripped all the fun out of art, that i gave up entirely for about three months. I would say have an overall monthly plan, but when that gets tedious, just pick up a pencil and do whatever you want, its better than nothing at all.
Hope that gobbledegook helps in some way, but im sure it won't! Lol. Good luck!