The anti Christ is in power for SEVEN years.
Revelation 6:16 And said to the mountains and rocks, Fall on us, and hide us from the face of him that sitteth on the throne, and from the wrath of the Lamb: 17 For the great day of his wrath is come; and who shall be able to stand?
The wrath has already started with the first seal.
Revelation 3:10 Since you have kept my command to endure patiently, I will also keep you from the hour of trial that is going to come on the whole world to test the inhabitants of the earth.
Faithful believers are kept from that wrath.
All of Revelation 6-18 is the wrath of God through which the body of Christ will not go through.
1 Thessalonians 5:9 For God did not appoint us to suffer wrath but to receive salvation through our Lord Jesus Christ.
With 483 years having passed from the decree to rebuild Jerusalem to the cutting off of the Messiah, this leaves one seven-year period to be fulfilled in terms of Daniel 9:24: “to finish transgression, to put an end to sin, to atone for wickedness, to bring in everlasting righteousness, to seal up vision and prophecy and to anoint the most holy.” This final seven-year period is known as the tribulation period—it is a time when God finishes judging Israel for its sin. Midway in that seven years the anti Christ sets up in the temple for 42 months. That is half of the 84 months of the tribulation.
>> “The anti Christ is in power for SEVEN years.” <<
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But his mark comes into effect only for the last 3.5 years. That is the great tribulation. That is when he stands on the mercy seat, and Satan assumes control, and he declares himself to be God.
That is when millions that refuse the mark either are starved to death or beheaded.
Really, there is no indication of exactly how long antichrist is in power, but we will not even know that he is antichrist until he performs the identifying abomination.