I bought a 30GB iPod photo in 2004, a 160GB Classic in 2007, a 120GB Classic in 2008. This year, nothing. I've seen nothing worth upgrading to. Maybe next year.
Updated from a 2005 mini to a 30GB video in 2006. (should have bought the 20GB 4G to begin with, the mini was too small to last me)
Since then nothing. I'll upgrade when either the mini or the video craps out beyond repair (and both have been repaired a couple of times) Actually before I buy a new one I'll probably stick some flash memory in the mini and start using that as my main iPod.
I'm more the repair than replace type And really they've never added anything all that compelling to the iPods since 2006.
I only upgrade my Ipod when I lose mine . That happens a lot though
So, it's normally every two years.
Anyway, yeah, here's my history of Ipods.
2004: 4G 20GB Classic-I had only, like, 100 songs on it at the time, though
2006: 1G 1GB Nano-Don't laugh, I still wasn't really "into music" at the time, so the size was just right
2008: 3G 8GB Nano-My all-time favorite Ipod
2009: 6G 80GB Classic-Just got it last week