I need advice

>How long does it take to do so

Literally until the day you die or quit art. Nobody just "gits gud" and stops, art is often a constant progression (and it's not always an objectively upward trend).

Composition and color wise I actually like your picture, OP. Zoomed in it's a little rough but it's a nice thumbnail.

Anyway, as general goals to maximize improvement, it's probably nothing you haven't heard before - identify your weak points (either through being reflective and self critical or by getting critiqued by others), and then study those things with books, tutorials, actual studies, etc. Rinse and repeat.
Draw regularly, get that mileage, and remember to do things for fun sometimes too. Even your fun drawings can be done with an idea of improving something. But go into a drawing with an idea of what you want to learn, and try to keep it precise enough that you can keep it in mind as you go.
The idea is to be conscious of your goals and actively stride for them. "Just drawing" will get you half way, for the other half you need deliberate study.

Additionally, make sure you hang onto your old drawings. What I do is arrange them monthly, and at the end of each month I put copy one or two of my favorite drawings for each month into a general folder. Comparing my favorites of a few months ago to my favorites of today can usually be enlightening.

>>2327507
>Nobody just "gits gud" and stops

Also just to nip this in the bud, inb4 sakimichan/kr0n/other comfort zone artists, you know what I mean and even those artists change little things here and there or polish their workflow.