Well, it depends upon the kind and content of the prophecy. When God's "covenant prosecutor," His holy prophet, pronounced God's judgment upon wayward Israel, the coming to pass of the prophecy given was not so far in the future that those to whom it was given and whose conduct had provoked God's wrathful prophecy could mistake its fulfillment. There are other prophecies, though, that were issued millenia ago that have yet to be fulfilled. But those prophecies were issued by men who were clearly marked and established by God as His prophets and whose fulfillment was not concretely anchored to a specific time. In those instances, then, where a prophecy clearly indicated a relatively immediate fulfillment of itself, a long forestallment of it exposed a false prophet for what he was.
These days, however, false prophets, have neither the miraculous signs establishing their right to prophesy, nor lives that are truly God-centered, rejecting sensual faith for spiritual and forsaking carnal pursuit of profit and power, nor do they prophesy in accord with Scripture, but add to it, or twist it out shape. And many of their prophecies have been time-sensitive, sometimes very time-specific, and have not come to pass. Even one prophecy of this sort issued from one claiming to be a prophet immediately dissolves their claim.
