Carol,
Every bottle has an expiration date printed somewhere on it. I am assuming that the manufacturers know something about how long their product should last.
However, common sense and logic would say that minerals will last longer than vitamins, and that any supplement that comes in a fat base won't last as long as a tablet. For example, vit. E (in a gelatin capsule) will go bad rather quickly, compared to mag-ox (which contains virtually nothing organic). I've opened a bottle of fish oil that was nowhere near it's expiration date, and it was rancid, but I'm betting that bottle of mag-ox will still be as potent (which is a joke in itself), years after the expiration date.
That being said, consider why you are no longer taking them. There must have been a reason, and chances are that reason still exists.
lisa