To understand what the church fathers said about eschatology, we first need to understand that they interpreted the scriptures differently than post-enlightenment scholars.
The Inspiration and Interpretation of Scripture: What the Early Church Can Teach Us
They believed there was a spiritual level of interpretation that was more important than the literal. For instance, Papias, a disciple of John, said:
Taking occasion from Papias of Hierapolis, the illustrious, a disciple of the apostle who leaned on the bosom of Christ, and Clemens, and Pantænus the priest of [the Church] of the Alexandrians, and the wise Ammonius, the ancient and first expositors, who agreed with each other, who understood the work of the six days as referring to Christ and the whole Church.
The six day six thousand year interpretation was common in the early church.
The Sabbath is mentioned at the beginning of the creation [thus]: “And God made in six days the works of His hands, and made an end on the seventh day, and rested on it, and sanctified it.” Gen. ii. 2. Attend, my children, to the meaning of this expression, “He finished in six days.” This implieth that the Lord will finish all things in six thousand years, for a day is with Him a thousand years. And He Himself testifieth, saying, “Behold, to-day will be as a thousand years.” Ps. xc. 4; 2 Pet. iii. 8. Therefore, my children, in six days, that is, in six thousand years, all things will be finished. “And He rested on the seventh day.” Epistle of Barnabas, Chapter XV.—The false and the true Sabbath.
Now we have understood that the expression used among these words, ‘According to the days of the tree [of life] shall be the days of my people; the works of their toil shall abound’ obscurely predicts a thousand years. For as Adam was told that in the day he ate of the tree he would die, we know that he did not complete a thousand years. We have perceived, moreover, that the expression, ‘The day of the Lord is as a thousand years,’ (Ps. xc. 4; 2 Pet. iii. 8.) is connected with this subject. And further, there was a certain man with us, whose name was John, one of the apostles of Christ, who prophesied, by a revelation that was made to him, that those who believed in our Christ would dwell a thousand years in Jerusalem; and that thereafter the general, and, in short, the eternal resurrection and judgment of all men would likewise take place. Just as our Lord also said, ‘They shall neither marry nor be given in marriage, but shall be equal to the angels, the children of the God of the resurrection.’ Justin Martyr, Dialogue with Trypho, Chapter LXXXI.—He endeavours to prove this opinion from Isaiah and the Apocalypse.
For in as many days as this world was made, in so many thousand years shall it be concluded. And for this reason the Scripture says: "Thus the heaven and the earth were finished, and all their adornment. And God brought to a conclusion upon the sixth day the works that He had made; and God rested upon the seventh day from all His works." This is an account of the things formerly created, as also it is a prophecy of what is to come. For the day of the Lord is as a thousand years; and in six days created things were completed: it is evident, therefore, that they will come to an end at the sixth thousand year. Irenaeus, Against the Heresies V.XXVIII.3
"And 6,000 years must needs be accomplished, in order that the Sabbath may come, the rest, the holy day “on which God rested from all His works.” For the Sabbath is the type and emblem of the future kingdom of the saints, when they “shall reign with Christ,” when He comes from heaven, as John says in his Apocalypse: for “a day with the Lord is as a thousand years.” (Ps. xc. 4.) Since, then, in six days God made all things, it follows that 6,000 years must be fulfilled. And they are not yet fulfilled, as John says: “five are fallen; one is,” that is, the sixth; “the other is not yet come.” (Apoc. xvii. 10.) The interpretation by Hippolytus, (bishop) of Rome, of the visions of Daniel and Nebuchadnezzar, taken in conjunction."
There are many other references to the six thousand years that you can find on the web.
Early Christian Writings
Augustine explained this views to the best of his ability.
The evangelist John has spoken of these two resurrections in the book which is called the Apocalypse, but in such a way that some Christians do not understand the first of the two, and so construe the passage into ridiculous fancies. For the Apostle John says in the foresaid book, “And I saw an angel come down from heaven. . . . Blessed and holy is he that hath part in the first resurrection: on such the second death hath no power; but they shall be priests of God and of Christ, and shall reign with Him a thousand years.” Those who, on the strength of this passage, have suspected that the first resurrection is future and bodily, have been moved, among other things, specially by the number of a thousand years, as if it were a fit thing that the saints should thus enjoy a kind of Sabbath-rest during that period, a holy leisure after the labors of the six thousand years since man was created, and was on account of his great sin dismissed from the blessedness of paradise into the woes of this mortal life, so that thus, as it is written, “One day is with the Lord as a thousand years, and a thousand years as one day,”( 2 Pet. iii. 8.) there should follow on the completion of six thousand years, as of six days, a kind of seventh-day Sabbath in the succeeding thousand years; and that it is for this purpose the saints rise, viz., to celebrate this Sabbath.
City of God, Book XX, Chapter 7.
also check;Of the Six Ages of the World.
On the Catechising of the Uninstructed — St. Augustine
The view is best expressed in the NT in Hebrews.
Heb 4:1 Let us therefore fear, lest, a promise being left us of entering into his rest, any of you should seem to come short of it.
Heb 4:2 For unto us was the gospel preached, as well as unto them: but the word preached did not profit them, not being mixed with faith in them that heard it.
Heb 4:3 For we which have believed do enter into rest, as he said, As I have sworn in my wrath, if they shall enter into my rest: although the works were finished from the foundation of the world.
Heb 4:4 For he spake in a certain place of the seventh day on this wise, And God did rest the seventh day from all his works.
Heb 4:5 And in this place again, If they shall enter into my rest.
Heb 4:6 Seeing therefore it remaineth that some must enter therein, and they to whom it was first preached entered not in because of unbelief:
It is related to the Jewish pre-Christian allegorical view expressed by Philo of Alexandria.
(16) What is the meaning of the expression, "Ye shall surely die?" (#Ge 2:17). The death of the good is the beginning of another life; for life is a twofold thing, one life being in the body, corruptible; the other without the body, incorruptible. Therefore one wicked man surely dies the death, who while still breathing and among the living is in reality long since buried, so as to retain in himself no single spark of real life, which is perfect virtue. But a good man, who deserves so high a title, does not surely die, but has his life prolonged, and so attains to an eternal end. Philo of Alexandria, QUESTIONS AND ANSWERS ON GENESIS, I*
Early Jewish Writings
Similar views can be found outside of Judaism in Hesiod's ages of man, the Pythagoreans ages of man and the ages of Zorastrianism.
The church fathers saw a more philosophical view of eschatology. Jesus said the scriptures were about Him (John 5:39, Luke 24:44). This was understood in a spiritual way as an embodiment of the Gospel in the church, the believer and mankind. Perhaps St. Methodius gives the best summary in his symposium speech of Thekla.