Updated: Nov 1
This subtle difference is one which I often see leads to confusion.
Let's look at a couple of examples to try and clear things up:
In this case the correct answer is how much farther.
Farther is used when referring to a physical distance so here, with a journey made in a car, the physical distance left to travel is the purpose of the question.
Let's now consider the following example related to careers.
Your career cannot be measured in a physical distance. It is a figurative distance since it measures how high you want to rise on the corporate ladder.
In this case "further your career" is the correct answer since there is no physical distance.
Further can also be used when talking about an addition to something. It's usage in this way would form sentences such as:
"Further to the point you made earlier"
"Further, it should be noted that..."