Let's say someone who is 35 has been an avid music fan since he was 15, and has bought 3 new CDs each week. I would find this scenario reasonable.

So 3 CDs * 52 weeks * 20 years = 3120 CDs
If you pick an average of 10 songs per disc, you're looking at more than 30,000 mp3s. Depending on encoding quality, I think that could require a bit more than a stock 60GB empeg.

I currently have just over 2100 songs on mine, most of which are encoded at 320kbps, but it's not even half full yet.