Assuming you are encoding music at 128bps that works out to approximately 1MB per minute. 256bps would be approximately 2MB per minute. That works out roughly to be either 8,000 minutes of music or 4,000 minutes of music.
Assuming each song is 5 minutes long, that means with 8GB of storage space, you can store approximately 1,600 songs @ 128bps, or approximately 800 songs @ 256bps.