"There are no legal rules permitting the use of specific number of words counts, a certain number of musical notes, or percentages of a work. Whether a particular use qualifies as fair use depends on all the circumstances." (Answer to Frequently Asked Question #47, U.S. Copyright Office Web page.)
The purpose of the fair use doctrine is to allow limited use of copyrighted material without requiring prior permission from the copyright holder. The statute lists four factors to be weighed when analyzing the proposed use in order to determine whether it is a fair one. Although consideration of all factors is required, all do not have to be in favor of a use to make it a fair one.
A fair use analysis is driven by facts and relies on your good-faith effort to make a reasoned conclusion. Reasonable individuals may come to different conclusions concerning the same set of facts, but the operative word is "reasonable." If you have made a rational and reasonable determination, you are less likely to be targeted for an infringement lawsuit.
Also see the section on Codes of Best Practice for the application of fair use within particular disciplines.
In determining whether the use made of a work in any particular case is a fair use the factors to be considered include: