Does anyone know how long a properly stored Mini DV tape will last before detoriation makes it unusable?

I don't really know, Mark. But I can tell you that I have some Beta tapes that have reached their 20th anniversary and still play fine. (Don't ask.) I also have some VHS tapes that are at least 17 years old that look as good as their youth (which, admittedly, was not dazzling by current standards.) So I would expect MiniDV tapes to have a long life if stored well.