How long will Christ's kingdom last when it becomes visible?
When Christ's kingdom becomes visible at His return, it will last a thousand years first; it will not be immediately eternal. Nothing in Scripture reveals this thousand-year time frame until Revelation, where God revealed it to John. It is the final revealing.
Les explains that Peter's sermon ties Israel's repentance to times of refreshing from the Lord. He notes it is a semicolon, not a period, so the thought continues into verse 20: if Israel would repent, God would send the refreshing in the person of the Lord Jesus Himself. But if Christ is sent back to earth, the time element was not given in the Old Testament or early New Testament; God kept it secret until He revealed it to John. When Christ returns and the kingdom becomes viable and visible, it would not be immediately eternal but would last a thousand years first. Nothing in Scripture indicates that thousand-year time frame until Revelation, the final revealing. Les also points out that Peter speaks of the kingdom promised back to Abraham, based on Israel's repentance and believing who Jesus is. Verse 21 shows a seven-year interlude Daniel spoke of in chapter 9, the seven years of tribulation, which is the times of restitution spoken of there.
And he shall send Jesus Christ, which before was preached unto you: