A friend of mine took on the 100 hand challenge that Disney interns have to take to better themselves at drawing hands. After seeing her improvements, I had to try it myself.
I drew ten hands per drawing session, each session taking about fifteen minutes. It took me two and a half hours in total to draw all one hundred hands.
My dad was a commercial artist and he said the human hand was really tough to render and do quickly and be correct everytime. I also worked at WDI for about 2 years and we had life drawing classes. The instructor put the most emphasis on drawing the hands & arm proportions.