There's no one right answer as to what constitutes a fair use of a particular copyrighted work. The answer varies from situation to situation.
Four factors are considered in all fair use evaluations. They are:
Purpose & character
Purpose: Is the copyrighted material being used for commercial or nonprofit educational purposes? Fair use favors educational purposes, but commercial entities can also take advantage of fair use
Character: Is the use of the copyrighted material transformative? (i.e. subjected to scholarly analysis, remixed, parodies, etc.)
Nature of the work
Has the work been previously published? Is the work primarily factual or creative in nature? Is this work created and/or marketed as a textbook or an educational consumable (i.e. workbook, etc.)?
How much of the work is being used? How important is the portion being used? If you are using the whole work, is it clear that no less than the whole work will be effective (i.e. photograph, poem, etc.)?
Is the work in-print? Is it available licensed elsewhere? Is there a market for the work? Can you identify the copyright holder? Did the copyright holder respond to a request for permission? Did you acquire this copy legally? Will you be able to attribute the original author?
These four factors are not meant to be exclusive and must be examined together. The statute does not indicate how much weight is to be accorded each factor, therefore, it is advisable to treat the four factors equally.
For help in making a fair use evaluation, please see the links below. The Columbia checklist is a printable PDF, while the American Library Association's Evaluator walks you through creating a fair use document for your records. In the event of a lawsuit, having such a document may help you prove you made a good faith effort to comply with the fair use clause of U.S. Copyright Law.
The College Art Association (CAA) provides visual-arts professionals with a set of principles and best practices in the fair use of copyrighted materials in scholarship, teaching, museums, archives, and in the creation of art .
The Association of Research Libraries (ARL) presents a clear and easy-to-use statement of fair and reasonable approaches to fair use developed by and for librarians who support academic inquiry and higher education.