Have a look at the iPod sales history on Wikipedia. It was in the 4th year with the 4G versions that iPod really took off.
Add in development time for the first version and that is something like 6 years to “Instant Success”.
Also note that Apple did a ton of work developing the Digital Music Eco-System to support iPod. Huge things like iTunes software, the iTunes store, the legal issue of getting songs on line, etc.