"Knowing when to quit is as important as knowing when not to quit. Hard work does not always pay off."

This is the entire subject of Seth Godin's _The Dip_. I'm not a huge fan of this breed of books, but if someone is looking for a lesson on this topic, I would having a look at that book. It's very short -- you can read it in under an hour. It provides a decent mental framework for thinking through the question of whether you should continue on your current path, or quit and switch gears.