@poisonedantidote That’s categorically false. There is no hard limit to the amount you can use for Fair Use to apply to you.
@Tropical_Willie Eight notes from a song may be infringement (I know nothing about Texas’s state copyright laws), but Fair Use is a defense to infringement. If you’re not doing something infringing, you won’t need to claim Fair Use. Nobody claims self-defense unless they actually hit someone else, right?
As for the question at large, the answer is “it depends” — it depends on what the song is, what part of the song you’re using, and what you’re going to use it for. The EFF has a really nice walkthrough of the Fair Use analysis:http://w2.eff.org/IP/eff_fair_use_faq.php if you’re curious. But without more information, no one’s going to be able to help out.
Also, I’m not a lawyer, I’m not your lawyer, this isn’t legal advice; I’m just a guy who did well in his copyright class in law school.