Out of the four common factors on a Fair Use test amount might be the most subjective. What consitutes the whole of the work? What amount can be considered small? Is the portion being used the heart of the work? Is the amount, however much that might be, approriate to how it is being used?
Figuring out what the whole of a work is problem that varies by format. The whold of a film is everything from the start of the film to the end of the credits. For a television show each episode can be thought of as a work onto itself. A song is a contained unit from the opening to the final note. Things get fuzzy with journals (is the unit a complete run, a volume, an issue, or an article), anthologies (is each portion by a seperate author a unit of its own or is the unit the collection itself), and books (does the book go from title page to end of index or is the book only the prepared content). None of these questions have solid answers.
Once you know the size of the work then the questions become is the amount you are using small and approriate to what you are doing. Small is a relative term that can only be decided once the size of the work itself is decided. How appropriate an amount is for your use will tend to be subjective and dependant on your purpose. However if you entend to argue that the use of an entire work is approriate to your project expect to have any Fair Use argument shot down. Generally it is best to take the absolute least amount needed.
The final question is did you use the “heart of the work”? Are you copying the portion of the material where the primary value lies? The heart of Gerald Ford’s autobiography was his thoughts on pardoning Richard Nixon, it was most people’s primary reason for purchasing the book. If a book is mainly a collection of publicly available statistics with a single chapter of analysis then that chapter is clearly the heart of the book. The most recognizable portion of a song or film may also be termed the heart of the work. Using this central part of a work is much harder to justify since it has a disproportionate effect on the value of the original work and our own work.
What can be said about amount is that the American Association of Publisher’s Classroom Guidelines are not legally binding. These guidelines attempted to set brightline amounts regarding books, poems, charts, and other works. However these guidelines do not take into account the overall size of a work. More importantly they are industry produced guidelines that can only be used as a best practices argument in a court of law. They are not part of the Federal Copyright Code and should not be treated as such.