Question: 
"Where do you get the idea that mankind has been appointed 6,000 years of self-rule to be followed by a 1,000-year reign of Christ?"
This form of reasoning has been penned for years, right or wrong.

Answer:

As Genesis shows, God reformed the earth and creat­ed the progenitors of all pres­ent life upon it in a six-day period, then rested on the sev­enth-day Sabbath. This began a weekly cycle in which man is to work for six days and rest every Sabbath (Ex. 20:9–11). In Hebrews 4:5–11, the Apostle Paul explained how the seventh-day Sabbath pictures the wonderful era of peace and rest that will follow the current age of man’s activity. In the book of Revelation, John was inspired to write that this coming era, beginning with Christ’s return to set up His Kingdom, will last 1,000 years (20:1–4)—often referred to as simply the “Millennium? (Non-Biblical term)

      

As the seventh day of the week, then, represents a thousand-year period in God’s plan it follows that the previous six days of the week represent thou­sand-year periods as well. In explaining what some would perceive as a delay in Christ’s return, Peter brought up this principle as something the Church should not be ignorant of: “But, beloved, be not ignorant of this one thing, that one day is with the Lord as a thousand years, and a thousand years as one day” (2 Peter 3:8 KJV).

           

The idea of each day of the week rep­resenting a thousand years of God’s plan was well-known to the Jews of Peter’s day. We now go to Jewish reasoning. About 200 years before Christ, Rabbi Elias wrote, “The world endures six thousand years: two thousand before the law, two thousand under the law, and two thousand under Messiah.” The famed his­torian, Edward Gibbon, wrote that “the tradition was attributed to the prophet Elijah” (Decline and Fall of the Roman Empire, p. 403). The Encyclopedia of the Jewish Religion (art. “Millennium,” Adama Books, 1986, p.263) reports that the tan­naim—rabbis of Christ’s day—based such an interpretation on Psalm 90, writ­ten by Moses: “For a thousand years in Your sight are like yesterday when it is past, and like a watch in the night” (v. 4). The tannaim said that, as there were six days of creation, the world would last for 6,000 years. The seventh “world day” would be 1,000 years of the Messiah (Sanhedrin 97a; Avodah Zarah Sa).

           

According to Gibbon, (Secular Reasoning)  the 7,000-year plan of God was “carefully inculcated” in the early Church. The extra-biblical Epistle of Barnabas (probably of Alexandria, not the apostle)—though it contains some errors—is illustrative of ideas prevalent at this time: “And God made in six days the works of His hands; and He finished them on the seventh day... The meaning of it is this.... in six days, that is, in six thousand years, shall all things be accomplished. And…when His Son shall come...then He shall gloriously rest in that seventh day.”

 

The “church father Irenaeus had been taught by Polycarp (disciple of the Apostle John). Sadly, he departed from apostolic teachings. However, he appar­ently retained some truth. In Against Heresies (c. 150 AD.), he related a belief of the early church: “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 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 years [end].”

 

To further illustrate how widespread the concept of the Millennium beginning six thousand years after Adam’s creation was, many more writings by other early rabbis and “church fathers” could be examined: Rabbi Ketina, Lactantius, Victorinus, Hippotylus, Justin Martyr, Methodius, etc. Though these men may not always be relied upon for biblical truth, they certainly do attest to how popular this understanding was in the early centuries after Christ’s death. This, in fact, has been the respected opinion of “Christian” scholars throughout the centuries, up to our present day.

 

As a final scriptural point, God told Adam that in the “day” he ate of the for­bidden fruit, he would die (Genesis 2:17). Yet Adam lived to be 930 years old (Genesis 5:5)! How is that possible? One way is just as Methodius and other early church com­mentators explained: since a day with God was a thousand years, Adam had to die before the first 1,000-year day was completed—and he did.

 

And all of this is how many have got the idea that there is a 6000 year plan for man.  etc.  1000 year reign of Christ.  All of this by Traditions of the Jews.