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2 hours a day is the standard time allocation. But more specifically, I'll say you need to sharpen at least 5 areas of creative nature:

Life Drawing - a source of reference : real life drawings of portraits, body postures, hands, feet, eyes & inanimate objects.
Art Theory - a place to draw some inspiration for your works : prehistoric art, egyptian art, greek art, roman art, modern art, etc.
Animation - a source of perspective : I know this forums is about manga (not anime), but animation is actually a very good source of perspective, i.e. when you make a turn-around. What is a turn-around? Self-explanitory. Make a character/ object turn around. Or make circles and lines move on a piece of paper. Or use a computer applicatoin.
Animation history/ manga history - a source for a source of reference : it helps to know a bit about your references.
Storyboarding - the concluder : applying what you know to comic style layout

Ok, so that was a little on criteria you should deal with.

Routine. As I said, 2 hours a day. Or if you can manage more, great stuff. Art is about discepline.

Anywho, if you have any further questions I'll gladly help.