Springs don't normally "wear" out. I could see that the ends of the coils may wear if there is no cover to protect it from contact with the strut, but this would likely not cause any significant problems. If the surfaces are kept fairly clean they should last a very long time. They may take set over time. This is typically caused by high temperatures (ones you are not likely to impart) and by heavy or over loads. The set does not affect the spring rate, only the ride height.
If you use used springs, check closely for rust pits, cracks, and/or nicks. Springs will begin to initiate a crack where there are pits and nicks. The time to failure depends upon how often the springs are cycled and how much load is applied.
