Today we ask the database query: When God decrees a punishment how much lag time does God give in order to give people time to repent? The query uncovers three major examples. An examination of these examples justifies the Rashi assertion that (a) The generation of the flood and Ninveh did not have leaders to learn moral behavior from; hence they were given extra time to repent (b) The Jews had great leaders to learn from and hence their punishment is immediate (Since no further time is needed to learn and repent) The table below presents results of the query along with illustrations of Rashi's comment.

Advanced Rashi: I have brought Ninveh as a supplementary example to Rashi. I would argue that Jonah's job was both to rebuke them that they would be destroyed as well as teach them proper behavior. Since Jonah was a temporary prophet-scholar (unlike Moses who was a permanant prophet-scholar) 40 days were given. By contrast the generation of the flood had no prophet to teach them - it is well known that Noah did not preach to his generation and the prophetic orders from God to him were personal not communal.