There are a couple of conditions that wear strings. One, being stretched (put on the guitar). Being under tension and being tuned contributes to metal fatigue and as the strings are used, they get stiff and although they will continue to stretch, they no longer stretch evenly. Hence, they get more difficult to tune and sound dead(er) because they are stiffening up. when the alloy fatigues enough, the string breaks. The other effect is corrosion. As long as the string is unused (still in it's envelope in the box) corrosion is minimal as the introduction of impurities is what speeds the process. Old string sets can still sound quite good if they have not corroded as they still retain the elasticity of non-fatigued alloy and have not corroded too much due to natural conditions. Strings being stored in an unsealed place near the sea could not be expected to last as long as a hermetically sealed set being stored in a desert hut. And everything in between....... The conditions that the strings are subjected to varies widely and the combination of the two major factors determines the useful life of the strings in question. Back in the day, I knew a fellow who could deaden a new set of strings in the time it took him to play one or two songs. His ph was not friendly to the string alloys. I have also noticed that strings on instruments that get played by several people (read: guitar shop demonstration guitars) age rapidly. Elixir strings (and the other coated strings) address that exact concern. But to the point: there is no reason why strings would not last almost indefinitely assuming they are stored properly, but all strings will wear out predictably once they are being used.
