I still have 3 Dell PowerApp100's, PIII 600 or 700's that have been running Redhat Linux, and Win2k for 4-5 years I believe.
Those have been rock solid even with being hauled around or shipped several times to new DC's. I recently sold off 1, pulled 1 for re-imaging, and still have one running a popular entertainment site and it's still doing great.
I don't buy non-redundant servers like those anymore, but their reliability has been impressive.
As drhamad said, Adequate cooling and clean air, plus clean reliable power are all you need to worry about for a server to run a long time (barring any quality control/warranty issues).
usually, 'moving' parts go out first in any server. nothing is more critical than HDDs which rotates at 5k~15k rpm. as preventive measure, I've customers insisting on changing HDDs every 2-3 years in mission critical sevrers regardless their condition
cooling fans are also subject to failure quite often, especially dust accumulation over years, the fine dust can get into the bearing and destroying the fans. changing out all fans on annual bassis is not a bad idea at all.
I've got a 1U server that is about 5 years old thats still running fine. I've had to replace the CPU fan and some of the other fans, but thats about it. I've upgraded the drive, but I'm sure that would've gone too. Its not a fast box, but its good enough for some basic junk.
1U server is basically several components working together such as:
- cpu & fan/heatsink
If a server is up and running for 1 week without any issues, then it is a good start. HD, cpu fan/heatsink and power are the items that would likely need replacing sooner than the other components. If the cpu fan/heatsink fails, you may want to keep an extra CPU just in case.
In general, HD should last 3-5 years as well as cpu fan/heatsink. If the power quality is good, then it will last at least 5 years in general.
If the motherboard is a good quality motherboard, then it will last a very long time. We have several servers that do not give any sign of stopping even after 7 years. We use mostly Tyan motherboards.
Temperature and air flow play vital roles in the longevity of your server too. Excessive heat will shorten the life of your server significantly.
So, in general if your server is of good quality and is running in a cool environment with good airflow, you should at least 5 years for 1U. The same thing applies to 2U or bigger.
Quality power supply, quality fans...and you will be ok.
If you get cheap and crappy fans.... if you colo it at some crappy datacenter with poor dust control or poor HVAC, your fans will suck in so much dust that your fans will die quickly or...you will have a 1U case of dust.