Nobody knows for sure, but I have good reason to believe that the final in-gathering will last a few decades with America as a key player, and the seven-year Tribulation will begin in the latter part of this century after Jesus comes for his saints. As I said, I believe this 21st Century will end with Jesus on the Throne in Jerusalem, the capital of the World, as King of kings and Lord of lords over all the earth.
'Nobody knows' because of the pervasive prophecy doctrines that are fake. This stuff circulates and then becomes established truths, which is why the 'watchers' will be the ones utterly blindsided. They are watching and anticipating... the teachings in the prophecy merchandise.
John 1:11 He came unto his own, and his own received him not.
"The prophetic perfect tense is a literary technique used in the Bible that describes future events that are so certain to happen that they are referred to in the past tense as if they already happened."
https://en.wikipedia.org/wiki/Prophetic_perfect_tense