The amount of music you can store on it depends entirely on the bitrate of the mp3 files.
Assuming 60 minutes of music ripped at 128Kbps:
(60 mins)(60 seconds/min)(128 Kpbs MP3s)/8 = 57,600 kilobytes
56,700 kilobytes/(1024 KB/MB) = 56.25mb
Using the above, if bitrate is higher than 128Kbps, then you need to go with a 128mb player.