How long does it takes to acquire a new skill?

   10,000 hours

Actually that is for expert-level performance. We don’t always need that.

How to be reasonably good from being grossly ignorant in a field? How long does it takes?

Answer is 20 hours : If you put 20 hours of focused deliberate practice into that thing you will be astounded with how good you are. Its just about 45 minutes a day for about a month, even skipping couple of hours here and there. You need to invest those 20 hours in the most effective way that you possibly can.

Method to do this:

1-Deconstruct the skill : Decide exactly what you need to learn and break it down into as smaller pieces as possible. The more you can break apart the skill the more easily you can decide what are the things that can actually get to what I want.

2-Learn enough to self correct : Get 3-4 books/DVDs/videos. Learn just enough that you can actually recognize your mistakes.

3-Remove practice barriers : Distractions, internet, phones..

4-Practice at least 20 hours