My current PC is probably 4 years now, but has been upgraded as much as I am likely to upgrade it now. Ram and motherboard and video card are original, but powersupply, case, CPU and a couple of other bits have been changed as I found suitable bits like a smaller HP case from my parents in law's computer they were getting rid of, a quad core CPU off trademe and a cheap Dell 23" monitor. Keeps it fresh and still fast enough.
Had an Acer laptop for 5+ years which only had a RAM upgrade. Just pulled together a system out of bits I had and a free case/mobo/cpu combo off here which is probably around 6+ years old except for a new GPU needed to get it going. That will do me fine for an ubuntu media center/media server. 
For work it is every 3-4 years, but being in tech services, we normally get the developers cast offs so we are a cycle behind. We just did a big hardware refresh though and got flash new I7 desktops with SSD + 1TBhdd though - yahoo!  
Loving the age of good enough computing. An old dual core system with 4+ GB ram and a decent GPU is still adequate for everyday use. Not good enough for gaming, but perfectly fast enough for everything else.