That's a bit like asking how long is a piece of string. The main thing to thing about is how much compression can I live with.
Most mp3s sound ok on rock music at around 128kbps if your taste is classical music then 192 kbps is what you should use. If you prefer wma then these limits can be lower as it has a more robust encoding system.
As songs and albums are different lengths, then a very rough guide would be about 10 rock albums per GB with mp3, so a total of 600 albums on the 60GB ipod.
That's quite a lot of music.