How long and how much practice does it take to learn a new skill?

Answer 1

Generally speaking, if it a simple skill, you will be learned and adequate at several weeks . Perfection takes longer.

Skills are classified into

SImple - cooking, baking, food preparation, sewing, ironing, painting, cash register, handling basic mathematical transactions, answering phones, postal sorting, learning a route, delivery, sanitation.

Intermediate- next level of simple skills where the job takes more knowledge and education added on to the manual labor skills, a category which also involves judgments.
Add on the months and years of schooling here.

Expert- Years of constant repetition of physical skills, honed muscles, habits now second nature or ingrained due to continuous hand-eye coordination, now in the teacher category.
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Answer 2

What skill are you talking about? And do you have any natural ability in that area? Have you already learned related skills? These all factor into the time it takes to learn a new skill. The other important factor that outweighs them all is desire.
So, I guess I'm saying there is no one answer to your question. Sorry.