The "15 Second" or "8 Bar" Rule
A common misconception among musicians is that we can legally use a piece of copyrighted material as long as we only use a small portion of it. The idea is that if you use just 8 bars of a composition or sample less than 15 seconds of a recording, you'll be protected. It's somewhat of an urban legend that appears to derive from the "De Minimis" argument that is sometime used in copyright cases, where one side argues that the copied/sampled portion is too small or common to be considered a distinct copyrightable work. The reality is that there is no legal protection in copyright law for these types of use. If you use a piece of a composition or sound recording that is copyrighted, you will need a license.
DE MINIMIS USE IN COPYRIGHT MUSIC LAW
Court Rules on the “De Minimis” Exception to Copyright Infringement
4 Music Law Myths That Indie Musicians Need To Unlearn