I'd guess at somewhere around 700 CDs-worth of music, assuming 128k variable bitrate. Assuming 4x encoding rate and a USB connection to the player I reckon you're looking at about 15 minutes per CD to encode and download, which means just over a week of solid work to fill it up. If we're talking about a real person then they're not likely to do more than a couple of hours of this per day, so in real life does that mean that the player wouldn't get full before a larger hard disk came out?
Sorry, just me in that Friday afternoon mood with a calculator handy. :)
Cheers,
Mark.