Well, what's the length of a song? You have to take into account the length of the mp3 and it's bitrate. I guess an average 128kbit/s mp3 would be aroung 4MB. You'd be better off working out how many minutes it can hold.
128kbit/s = 16kbytes/s
192kbit/s = 24kbytes/s
256MB = 262144kbytes...