There's a widespread myth in the sampling community that "less than two seconds is fair use." Don't believe it. What a judge and jury will feel is fair use depends on a number of factors other than the length of the sample. Generally, when reviewing fair use questions, courts are looking for three things:
You did not take a substantial amount of the original work.
You transformed the material in some way.
You did not cause significant financial harm to the copyright owner.
Also, some courts only apply this fair use rule to the musical composition copyright, not the sound recording copyright.
In principle, it's good to know these defenses, but the obvious difficulty with all of them is that they are defenses. The time you'll use them is when someone is coming after you. There is no predictable way to guarantee that you'll win your court case based on these defenses (assuming you can even afford to hire attorneys to fight the case).
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