I upgrade something roughly annually; but its rarely a full system unless I hit a CPU bottleneck. It's been a bit over 2 years since I bought my last processor, so probably 80% of my current build is from then. However, 8 months after that I upgraded my 3d card. And back in November I upgraded all my monitors.

Most of my upgrades fall into the category of 'things are working pretty well....but they could be better.' That said, most were aimed at fairly concrete goals, e.g. going from hardware to software, going from 5 to 10, backing off high CPU usage while in game, bigger monitors, or attempts to improve system stability. Those are much better metrics than 'has tech doubled' IMO.

If money really isn't an issue, I say go for it.