Having experience from the healthcare side of things...
Four years is a good average. As long as the temp is between 50 and 60 degrees F, that is a good estimate. Higher than that and you can halve the shelf life. Below 35(aka, freezing) and they will last for about 1 month if there is a freeze/unfreeze/refreeze cycle.
We usually recomend looking at the pakaging to see what condition the condom may be in. If the packaging is delaminateing, then religate the condom to non preventative uses and get fresher condoms. If there are bubbles, peeling or discoloration, get new condoms. If you can see the ridges pressed into the packaging from the condom, get more condoms.
The one type of condom that has a good shelf life that I can personally recomend for many uses is the Gold Coin veriety. They come non-lubed, can easilly be repackaged and come in really cool foil colored disks. And in the outback, that foil could SAVE YOUR LIFE! <img src="/images/graemlins/wink.gif" alt="" />
Latex and 'natural' condoms will have a shorter shelf life than the poly ones out there, though I have not found a non-lubricated poly condom, yet.
Non-preventative uses include finger cots(multiple fingers), gun caps(over the muzzle/barrel), water storage, clean skins for adult toys, water bombs...
Rena