The idea of a thousand-year reign of Christ – a millennium – is found only in Revelation 20:4, 6. The length of the martyrs’ or saints’ reign is here said to be a thousand years. This number has produced the term "millennium," which is derived from the Latin mille (thousand) and annus (year).
Jewish apocalyptic writings of the time when Revelation was written and before speculated about the length of the Messiah’s reign, when it was assumed the nation of Israel would be restored to glory by God. The time ranges ran from 40 to 7,000 years. The author of 4 Ezra thought the Messiah’s "millennium" would last 400 years (7:28). That means the hearer-readers of Revelation probably would have been familiar with the Jewish idea of a limited reign of Christ.
The writer of Revelation may have mentioned the "millennium" to counter the idea that the "kingdom of God" referred to in the Hebrew Scriptures was to be based around a physical Jewish nation. An important point of Revelation is to reinterpret Old Testament prophecies in terms of Jesus’ redemptive work and the church. It’s helpful to understand that Revelation was written to point out that the church was the recipient of God’s grace, made possible by Jesus’ saving work. The book’s message to the church contradicted the Jewish idea that salvation would be national and come to the Jewish people alone. Given these factors, it’s not surprising that Revelation would make a comment about Jewish millennial speculations and expectations, and reinterpret them in terms of God’s real purpose in terms of the church.
We also have to distinguish between some Jewish ideas about God’s ideal kingdom and what the Old Testament says about it. The Old Testament says nothing about the Messiah’s rule being a millennium in length or that it would last for a limited time. It seems to speak of the kingdom of God on earth as being open-ended or continuing without end once it begins. Even the kingdom of the "new heavens and new earth" in Isaiah 65:17-25 and 66:22-24 appears to be but an extension of the earthly and seemingly eternal reign of the Messiah, about which the prophets spoke.
Neither does the New Testament directly speak of Christ’s kingdom as existing in any limited time sense. The only passage that might indicate a time-limited kingdom as existing between Christ’s coming and the beginning of a more glorious kingdom existence is 1 Corinthians 15:22-24. Paul here seems to speak of "the end" as being in some way distinct in time from what apparently is his reference to Christ’s return. If that is so, Paul gives no specifics. In none of his writings does he express any interest in or undertake any discussion of a limited "millennium." Neither do the other New Testament writings. We also should note that the concept of "the end" is also understood in the New Testament as that time that begins with the completion of Jesus’ work of redemption.
The mention of 1,000 years in Revelation – a book of symbolic numbers – forces us to ask whether this period of time is literally 1,000 years, or whether it is even to be taken as a limited period of time at all. Any attempt to answer this question must rely on the context of Revelation 20, for no other Bible verse clearly discusses such a period of time. But if we attempt to use a single passage in a highly symbolic book as the basis of a dogmatic conclusion about a theological idea or doctrine, we are violating a cardinal rule of biblical exegesis.
Considering those limitations, some commentators nevertheless believe the figure given in Revelation 20 represents a literal 1,000 years. Other biblical commentators feel that while the "millennium" is a real period of substantial length, its actual time is undetermined. In the same way that "one hour" means a very short time (Revelation 17:12), 1,000 years would mean a very long time.
Those who feel the number "thousand" refers to an indefinite though long time cite examples of similar usage from the Old Testament. In Psalm 50:10 God speaks of himself as owner of all that exists. He says, "Every animal of the forest is mine, and the cattle on a thousand hills." Obviously, the expression is not to be taken literally, as though God owned cattle only on 1,000 specific hills.
Job 9:3 speaks of man’s inability to box God in with arguments by saying, "Though one wished to dispute with him [God], he could not answer him one time out of a thousand." What he means is that in any dispute with God, we humans lose the argument because his wisdom and understanding is infinite and ours is very limited.
In the New Testament, Peter says that with God one day might just as well be a thousand years and a thousand years a day (2 Peter 3:8). That is, what we think of as a long time, to God is but a very short time. Again, it is a metaphorical way of expressing the idea that time has no meaning for God so that we need to understand the significance and timing of human events from his perspective, and not ours.