Copyrighted Song Length On A Podcast

If you use a song as Fair Use on a podcast, how much of it are you allowed to play? My friends think it's 30 seconds, but I've never seen that posted anywhere on the web, so I'm not sure.

I studied broadcasting: 30 seconds or 10% of the overall length.

http://en.wikipedia.org/wiki/Wikipedia:Music_samples

whichever is less*, just to clarify what gnomey meant by "or".

And yes, I am going to keep calling you that.