And the LORD said, "My Spirit shall not strive with man forever, for he is indeed flesh; yet his days shall be one hundred and twenty years." - Genesis 6:3

This generation will not live nine hundred years like the previous generations, for it is flesh and its days are filled with the deeds of flesh. Therefore, their days will be one hundred and twenty years. If they repent during this time, they will be saved from the wrath that is about to come upon them. But if they do not repent, by their deeds they will call down wrath upon themselves. Grace granted one hundred and twenty years for repentance to a generation that, according to justice, was not worthy of repentance. .

- St. Ephrem The Syrian

Fr. Rose in his book, "Genesis, Creation, and Early Man" expounds on this citing plenty of other Patristic sources.

I too did not know this, I cannot recommend this book enough.