Had mine for coming up for two years (in June) so Applecare lasts till June 2009, if the computer doesn't die on my I would love to say I will get a new MBP in June 2010 or 2011.
For me 4 years would be fantastic, but it is less about the speed of the new machine and more about how much money I am earning at the time to justify the new purchase.
I would estimate that within 6 months of the next version of OS X (10.6) I would need to upgrade to keep my productivity up. When the next version of the OS comes out is a mystery, seems like a long way off after such a great version of Leopard came out just 6 months ago.