Over the years I tried different upgrade schemes, and it seems 2 or 3 years works best for us. Over the course of that timespan there is an overall performance gain large enough to notice and worth money. If you go with less, say 1 year, the gain is much smaller, and you essentially pay for something that's the same but just looks newer. Also depending on the country you live in, there's a certain amount of time you can use hardware as an actual cost on your tax form. As such keeping it much longer than that time means loss, keeping it much shorter as well.
The psychological aspect (which we all know since childhood already and basically is set in our genese: new = interesting) does do something, of course, but has in my experience no long-term effect whatsoever apart from it costing money. Especially with short upgrade cycles the 'wow, new' effect gets less and the money effect more.