Much of my mp3 collection is encoded at about 128Kbits/sec (yeah, I know). That works out to about 1MByte/minute of play time. So.. 30 minutes gives 30MByte/day, times thirty days, is about 1GByte/month.

If you are skipping through tracks, or using higher quality encodings, then just multiply by 2 or 3, and there it is.

Cheers