I love the challenge of painting a subject in one sitting but when that subject is a person it adds to the challenge. It's a balancing act of color and value while maintaining proportions and correct drawing. It's an intense undertaking.
On a related note I will be putting together some tutorials / demos on painting in oil. If you are interested in learning how to paint or are just looking to sharpen your skills please take a moment to comment on what you would like to see in these tutorials. Your comments are appreciated!