"Upgrading" is also an issue of "turnover." If you're adept at moving used parts frequently for resale at EBay, upgrades may be more frequent.
If you're a nitpicker about your home-LAN infrastructure, data, cost of software licenses and other such things, you may hold onto that old Q6600 (or Nehalem . . . or Sandy Bridge) for a lot longer. Think of all this in a context of several household systems.
Take my server, for instance. It's an old 680i board with a Q6600 I picked up last year for $80. The board had been in storage for five or six years -- I merely forgot I had it. And as for the WHS-2011 OS, I'm going to squeeze blood out of a stone before I'm ready to replace it.
We're all part of a mindset here. We're all computer-obsessives. We can't help but think what wonders are around the corner with each new processor and chipset generation.
I have this perfectly good E8400 in a P45 board and 8GB RAM. For the last six months or so, it looked like it was slowly going south. I finally discovered that even Seasonic PSU's die of old age. But I'd already offered the CPU, board and half the RAM to a friend who can use it.
It's time to phase out LGA-775, but I'm going slow on this . . . about that . .