It used to be 30 seconds.....since new media has enter the market the "legal" answer I got from a copyright attorney is that if you play copyrighted music, you owe royalties. It doesn't matter how long it's played. Fair use does NOT apply, unless of course you're going to discuss the piece being played (giving critique, etc.). If it's used for bumpers, then you owe royalties. There's a famous case where an internet station was sued for playing just THREE notes from a popular song. They were sued, they lost and had a judgement in the thousands.