Have you heard about the mythical "30 second rule" or a "10 second rule" for fair use? According to the Student Press Law Center's Guide to Music Licensing (broadcasting and webcasting), these have made many people think they can use shorter musical clips for their purposes without the need to obtain a license. No such rules exist in copyright law. While courts consider the amount of work used, this is only one of four factors courts consider in evaluating fair use. Even with a few seconds, a court could still find factors that weigh against fair use. The Supreme Court has stated that even if a use is minimal, it could constitute infringement if it takes the "heart" of the work/song.
For more information, see http://fairuse.stanford.edu/overview/fair-use/four-factors/
Fair Use Guidelines For Digital Images provides useful information for assessing fair use of digital images.
The Center for Social Media in the School of Communication at American University, the Program on Information Justice and Intellectual Property in American University Washington College of Law, and the Media Education Lab of Temple University conducted a project 2007-2009 to clarify fair use in media education, with support from the John D. and Catherine T. MacArthur Foundation. This project will help media literacy educators understand their rights under the doctrine of fair use in order to help them more effectively use media as an essential part of their teaching.
Under the “fair use” rule of copyright law, an author may make limited use of another author’s work without asking permission. However, “fair use” is open to interpretation. Fair use is intended to support teaching, research, and scholarship, but educational purpose alone does not make every use of a work fair. It is always important to analyze how you are going use a particular work against the following four factors of fair use.
U.S. Copyright Office provides a fact sheet
The following two charts can provide helpful information on deciding if you are using copyrighted material fairly.