Not so much replaced because they no longer perform as required, require too much maintance or constant upgrading to fitt the ever increasing needs of a business they are replaced purelly for taxation reasons.
Currently we depreacate all computer equipment 30% per year and when a server or any computer equipment gets down to a value that is detirmed by the accounting department as no longer being cost effective for taxation purposes they are replaced. Every server that I have pulled off line and replaced was working perfectly and in all probability would continue to do so for a very long time to come and with the exception of things like ISA Servers all our production Servers are multi processor units with inbuilt SCSI so they are upgradable for a very long time I would imange it all depends on what you term as a server.
The servers that I work with are run 24/7/356 and very rearly cause any problems other than self induced ones but they basicly run all the time with proper protection builtin ad we only go aff line when something like the Slammer Worm or something similar creats a DoS which incidently didn't affect us. Of course we routinely pull a server for maintance but we run a redunt sustem where I work so one going off line for a few hours doesn't present much of a problem if any and we never pull a server durring peek ussage times.
As a guide if you have good servers warranty isn't an issue with us and just look at what your accounting department has devalued these units to and use that as a guide as to when they need replacing. You are in a better position that anyone else here to know what exactly you have but all the application servers should have as a bare minuinum a Dual Process M'Board at least 1 GIG of RAM and two SCSI channels that can handle up to fifteen devices per channel to support all your required drives and give you room to grow as required.
If the things are constantly failing you have a very basic problem as these units wern't fit for the job that they where bought for originally or lack the proper protection devices these are both hardware and software. Although you will never have a totally secure system that is invunerable to attack it should at the very least be somewhat secure and here is something very important a lot of problems arrise when staff are allowed to access the network from home as their home computers don't have the necessary security procedures installed so this could be a source of your problems as we now not only secure the network but also every persons home computer who has the authority to access the network and while at first it proves expensive in the long run it saves a heap of problems. Particuarly with CEO's and Board Members but every home computer that has access to our network is not only secured at their residence but we also perform any repair work required just to keep our network secure and not allow any other unorthirised person access to a means of accessing our network. This used to be our buggest problem some CEO would take their computer in for repair and then that would give the repaired or one of their staff the means to access our systems with all the privilages of the nominated user. We only picked this one up when one of the Board Members was accessing the network while in hospital in a coma so we knew it wasn't him so we locked the whole system down and when we backtracked we found that his computer had been taken in for repair and his HDD replaced or cloned and all his access priviliges had been stolen buy someone else, we never found out who but now we perform every repiar job imangiable from changing a LT battery to decomissioning computers.
If you want any more details feel free to e-mail me direct.
