Why did God give Nineveh 40 days?

God gave Nineveh 40 days as a period of warning and opportunity for repentance following Jonah's prophetic message of impending destruction. This timeframe allowed the people of Nineveh to reflect on their actions and turn away from their wickedness. The 40 days served as a divine demonstration of God's mercy, giving them a chance to seek forgiveness and change their ways before judgment would be enacted. Ultimately, their repentance led to God's compassion and the sparing of the city.