Student Press Law Center: Guide to music licensing for broadcasting and webcasting
"Students and advisers frequently ask ... about a "30 second rule" or a "10 second rule" for fair use. These mythical rules supposedly allow use of music or video clips no longer than the specified time (30 seconds, 10 seconds or some other fixed limit) without the need to obtain a license. Unfortunately, there exist no such rules in copyright law. Although courts do consider the amount of the work used, this is only one of four factors courts consider in evaluating fair use. This means that even if the clip used amounts to only a few seconds, a court could still find that the other factors weigh against a finding of fair use. Additionally, the Supreme Court has stated that even if a use is minimal, it could still constitute infringement if it takes the "heart" of the work. This means that a clip of even a few seconds could be infringing if a court finds that the clip has captured the "heart" of the song."