Who knows whether God will not turn and repent, and turn away from his fierce anger, so that we might not perish?"
Some have supposed that the Ninevites had no hope when they turned to repentance, but that is inaccurate.  God had given them the "sign" of the prophet Jonah (Luke 11:30); and they knew that Jonah, in the very midst of his rebellion against God, had nevertheless received mercy, and they may well have surmised that it could be even so with them.  There was also the matter of the forty days promised by the Lord before the destruction; they evidently understood this accurately as an opportunity for them to amend their ways and appeal for mercy.  Otherwise, God would have destroyed them instantaneously, without any time lapse at all.