What happens after the undetermined period of time?
After the undetermined period of time ends, the seven years will come in. Then Christ will return, and then the King will come and set up His Kingdom. No one knows when that undetermined period will end, but the sequence is fixed: seven years, Christ's return, and the Kingdom.
Les explains that because Israel kept rejecting the offer of the King and the Kingdom after Christ ascended, God moved all of this into the future and inserted an undetermined period of time. No one knows when it will end. Then the seven years will come in, then Christ will return, and then the King will come and set up His Kingdom. Les notes that Peter and the other eleven operated only on what God had revealed—the Old Testament program—and had no concept of God turning to the Gentiles for 1900 and some years. This was not their fault, because God had been holding it a secret and was giving Israel every opportunity to repent. In the early chapters of Acts, Peter isn't talking to the church or to Gentiles.