When was a thousand-year time frame first attached to God's Kingdom?
The thousand-year time frame is first attached to God’s Kingdom in Revelation, where it is called the Millennium. Until then, the Old Testament, Christ’s offer as King, and Paul’s references speak of the Kingdom without a time frame, while Revelation sets its length at a thousand years.
Les explains that all through the Old Testament, the Kingdom promised to Israel had no time frame associated with it. Even when Christ came and offered Himself as King, and even when Paul wrote about the Antichrist and the things pertaining to the end of the age, nobody put a thousand-year time frame on the Kingdom until Revelation. There it is called the Millennium. Les also notes this does not contradict the Old Testament promises that the Kingdom is forever and ever, because the thousand-year reign of Christ is really an introduction into the Eternal. The millennial reign will end, with a complete change in Creation and the Universe, but eternity will be on much the same basis as the Kingdom: nations again, Christ ruling and reigning, perhaps from the New Jerusalem. The thousand-year reign will be another time of testing, since those entering the Kingdom are only believers—Gentiles who believed during the Tribulation under the 144,000’s preaching, and the remnant of Israel will be saved.