It depends on what compression and file format you use. That usually refers to 48kbps compression whereas a usual mp3 is 128kbs.
And if you want something with a huge battery life get a Sony NW-HD3. It has 30hr battery life and can take 13,000 songs if you use Atrac3 but it can take 5,000 normal mp3s as well if you don't want to use Atrac3