I am interested in realistic sketching and would like to draw more accurately. How long does it take (to learn)? Can human anatomy be studied independently?
The short answer: It honestly depends on many factors.
The long answer: Many factors play into it. It is good to know anatomy and life drawing. It is a MUST if you want to draw realistically, be an animator, draw superhero comics and so on. Even if you're not pursuing those avenues, anatomy helps a lot. The human body is very hard to draw; it is said that if you can draw humans, you can draw anything. That's why art schools spend so much time on life drawing.
It can take a few weeks to a few months to get the basics of anatomy down but it takes a lifetime to master everything (if you can do so). It depends on your own drive. To that end, look around for instruction. Check art schools or art galleries for workshops. Failing that, there are a goldmine of books at your local library about drawing people. Study them. Also draw a lot; draw from photographs from online or magazines. Take a sketchbook with you to the mall, the coffeehouse, the airport, etc and draw people you see. You only learn anatomy by doing it.