There's no set portion of a song that takes it from "fair use" (essentially how it's "okay" to do this) to infringement. The four factors judges consider are:
- the purpose and character of your use
- the nature of the copyrighted work
- the amount and substantiality of the portion taken, and
- the effect of the use upon the potential market.
Using portions of it for comment, criticism, education, etc... will be considered fair use if these other factors, taken together, indicate that fair use is allowed. I tell customers that if people will be downloading your podcast so they don't have to pay to download the song, then that would obviously be infringement. That's the "effect of the use upon the potential market" which is most important from the standpoint of the owner of the work (and thus most likely to make them make the decision to sue for infringement if it comes to that). Short portions (10-15 seconds apiece, maybe up to 30 seconds) would be what you should be aiming for, and only in rare situations go beyond that, but not more than a minute. That's a significant portion of the copyrighted work, and would probably not fall under fair use.
Hope that clears things up a bit. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable.
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