How long is unused coolant in storage good for? Coolant being used should be flushed about every two years, so should coolant that has never been used be disposed of every two years also? Thanks.

No. The shelf life of unused coolant can be many years, even if the container has been opened and some of the contents used. When coolant is in your engine it is subject to extremes in temperatures and also byproducts of combustion. The protection packages that are in the coolant is what can break down over time when it is in your engine as it is subjected to the items above that I described, and that is why you need to replace it according to the manufacturers recommendations.