St. John Chrysostom, Hom. 34 on John, 3:
If three hundred years ago, when those words were used, Paul called that season “the fullness of time,” much more would he have called the present so. But perhaps for this very reason some disbelieve, yet they ought on this account to believe the more. For whence do you know, O man, that the end is not “at hand,” and the words shortly to be accomplished? For as we speak of the end of the year not as being the last day, but also the last month, though it has thirty days; so if of so many years I call even four hundred years “the end,” I shall not be wrong; and so at that time Paul spoke of the end by anticipation. Let us then set ourselves in order, let us delight in the fear of God; for if we live here without fear of Him, His coming will surprise us suddenly, when we are neither careful, nor looking for Him.
St. Augustine, Eighty-three Different Questions, qu. 58, 2:
Now old age usually lasts as long as all the other ages together. For since old age is said to begin with the sixtieth year, and since human life can reach one hundred twenty years, it is clear that old age alone can be as long as all the other earlier ages. Therefore, in regard to the final age of mankind, which begins with the Lord’s coming until the end of the present world, it is uncertain how many generations are reckoned to it. Moreover, God has willed for our benefit to hide this, as it is written in the Gospel and the Apostle attests, saying that the day of the Lord will come as a thief in the night.