USE OF COPYRIGHTED MATERIAL
(From the website of the U.S. Copyright ofc.)
How much of someone else’s work can I use without getting permission?
Under the fair use doctrine of the U.S. copyright statute, it is permissible to use limited portions of a work including quotes, for purposes such as commentary, criticism, news reporting, and scholarly reports. There are no legal rules permitting the use of a specific number of words, a certain number of musical notes, or percentage of a work. Whether a particular use qualifies as fair use depends on all the circumstances. See FL 102, Fair Use, and Circular 21, Reproductions of Copyrighted Works by Educators and Librarians.
http://www.nolo.com/legal-encyclopedia/fair-use-rule-copyright-material-30100.html
The ‘Fair Use’ Rule: When Use of Copyrighted Material is Acceptable
In some situations, you may make limited use of another’s copyrighted work without asking permission or infringing on the original copyright.
Under the “fair use” rule of copyright law, an author may make limited use of another author’s work without asking permission. Fair use is based on the belief that the public is entitled to freely use portions of copyrighted materials for purposes of commentary and criticism. The fair use privilege is perhaps the most significant limitation on a copyright owner’s exclusive rights. If you write or publish, you need a basic understanding of what is and is not fair use.