“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.’ 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

“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

Just a couple writings. There are obviously more that you can find.