In order to draw a good nude, some drawing skills are necessary but it is not enough. If you rely only on your eyes, chances are your drawing will look messy, disproportionate, and plain wrong. You will often need a basic knowledge of anatomy, such as the arrangement of the bones in the skeleton. This way you can see the model from the inside as well as the outside, helping you to understand what you see and hence, draw it better.
On top of that basic knowledge it is often very helpful to know the main muscles in the body to understand what happens under the skin of the model as he or she takes different uncommon postures. Without this knowledge it is unlikely that you would be able to resolve an awkward perspective of, say, a foot. However, this skeleton frame is not to be followed to the letter: some models are disproportionate according to the standards of the day : very long limbs, huge heads or very short torso. The anatomical knowledge is no substitute for acute observation.
A pose can last from 10 seconds to 30 minutes in a 2 hours session. Bearing in minds that there is a point in your drawing when, whatever you do, you loose the spontaneity of the first shot and start to degrade your creation. One of the thing you must keep in mind is to know where to stop. That is true for any kind of drawings. Depending on the course you are following and your intention as an art student you may need the same postures for days on end, even weeks or more, especially when doing a sculpture, although it becomes increasingly rare to encounter truly anatomically correct nude drawings and sculptures.
