There are systems that have been running for over a decade, there might be some part failures along the way like a dying hard drive or a failed optical drive, but i have some systems from 2000 that are still running fairly well, and a PII in my basement that runs just fine last time i check, if its taken care of most things in a computer have a very long lifespan.
I have a working 700mhz hp mini atx box that I occasionally boot to play old dos games on that I cant get to run on win7. If it wasnt so slow I could probably have kept my old 486 dx2/66 running but that might be a stretch.
Now if you were talking gaming and keeping up with the latest game graphics you will be looking at upgrading at least the video card each year or so. Even then the case (if you get the right one from the get go) and the ancillary components (i.e. other than ram, cpu, mobo, psu and video card... maybe hard drive these days) should last at least 5 years. Ive still got a full tower atx case running from 2003 that is a tad small for todays aftermarket coolers but it works fine for stock coolers and is still long enough to put even a 580 in.
All In all though if you want to build a system and you build it right and dont get into mad over clocking or tinkering with it every couple of days (Like I do) it should last you quite a while... 10 years maybe if you leave it alone and format the hard drive every so often. Just make sure you get a good power supply so it doesnt die on you and you're good to go.
Well I ran a P4 2.8Ghz 1.5GB ram from about 2002 to 2009 and the only thing that ever failed in the system was a power supply it was a cheap startech power supply though, but I did switch video cards about 3 times in that time period. (ATI 9000 pro/ATI 9600/and X850XT)
And I still have a P3 800Mhz 1GB ram system running Windows Xp in my spare room that still works just fine. Think I got the P3 system back in 1999 to replace my Mac (performa 5260). Of course though its not really good to run todays programs very well, its really only good for email and browsing the internet. And the only thing to ever fail on my P3 system is a Maxtor hard drive, I remember freezing the drive to try to retrieve some data off it before it completely died.
So yea you can make a system last a long time if you take care of it properly and protecting your system. And I don;t think you need to upgrade your video card every year, even if you are a gamer, I had a ati 850XT for about 4 years and ran games just fine. So no you don;t need to upgrade a video card every year or so, its more of a "want"
Although I still have both of these system neither one really gets used anymore, the question comes down to how long will the system preform to your expectations. If your a hardcore gamer or performance junky your going to want to upgrade a lot. If your just a casual gamer/surfing the net/email/ you probably won;t upgrade to often. But again some hardware does fail over time no matter how well you take care of it.
And I agree with re-formatting and reinstalling windows every once in awhile, theres nothing like a system with a fresh install.
I've been on the same case and motherboard since 2008, but everything else in my PC has changed since I've first put it together ^_^. Anyway, I'd say an acceptable usable lifespan is 5-6 yrs depending on the demands placed on it. Some businesses refresh their PCs every 3 or 4 yrs since they often keep them on 24/7 and they actually want their employees working rather than waiting on the computer that's struggling with a shinny new program. My job has PCs that are 10 yrs old still humming along and some are barely hanging on O_o.
My current build is from 11/2003 and still runs great. I did upgrade HDD and GPUs over its life span. It is a P4 HT. I am getting ready to replace it this week with a new i5 2400 system. I have an old AMD dx4 120 someplace in a box as well that is 15 years old. I am sure if I powered it up it would work just fine. I think it has Window 95 on it. I recently found my old MS DOS and Windows for Workgroups disks and a memory manager. Toying with idea of rebuilding it with these older OS just for the fun of it. Not sure if I am in the mood to try relearn how to manually configure the IRQs.
I am a big gamer and i usually do a major upgrade every 3-4 years. Often i will have to replace something in that time.
i also really enjoy gaming,my new pc is a core I5 2500k p67 pro 3 board cm 212 cooler gtx560 gigabyte graphics antec 650 watt true power,that was my plan to rebuild around every 3 years,its a hobby i enjoy.
I had a 12 year old P233MMX system that I finally discarded, because I wasn't using it and I couldn't give it away. I have a 500 MHz P3 system that I might be able to give away, but I doubt it. I used an inexpensive AMD build for almost 7 yearsbefore I built my first Core2 system.
Short of a random failure and assuming you stay within manufacturer's recommended voltage and thermal limits, your PC is more likely to become obsolete before you have a hardware failure.