Final answer:
The biblical prophecy mentioned in the Book of Jonah states that Nineveh was given 40 days to repent before it was to be destroyed by God.
Step-by-step explanation:
The question pertains to the biblical prophecy regarding the destruction of Nineveh, an ancient Assyrian city. The historical number of days before Nineveh would be destroyed by God is not detailed in historical records but is mentioned in the Bible. According to the Book of Jonah in the Old Testament, God gave Nineveh 40 days to repent before it would be overthrown. Hence, the biblical answer to the student’s question is 40 days.