The frequency of upgrades depends on your needs. I don't play games on my computer, so I rarely upgrade the video card, going on 5 years since the last upgrade and that was with a mid range card at the time. I do quite a bit of video processing so I tend to upgrade the CPU fairly often, about once every 2 years though the 2600K has easy overclocking potential so it becomes more like 3 years instead meaning my next upgrade may be 4 times as powerful than I have now (given Moore's Law). Hard drives are upgraded about once every 3 years, mostly for improved read and write speeds. Of course, the end result depends entirely on what your needs are. My case is just an example on how I decide when to upgrade.