
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.
A good point to consider is this: Have you made a "good faith" effort to comply with the "fair use" clause of U.S. Copyright Law?
"Four factors" are considered in all fair use evaluations. They are:
1. Purpose & character
2. Nature of the work
3. Amount
4. Effect
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. Historically the courts have placed the most emphasis on "effect", while the "nature" of the copyrighted work is usually considered to be the least important factor.
An excellent resource to use is the ALA's Fair Use Evaluator.