How We Know The Tribulation Will Last Seven Years

Daniel 9:27, "And he shall confirm the covenant with many for one week: and in the midst of the week he shall cause the sacrifice and the oblation to cease, and for the overspreading of abominations he shall make it desolate, even until the consummation, and that determined shall be poured upon the desolate."

The "one week" must be 7 years, because "in the midst of the week" the Antichrist breaks the peace in Israel, which we read in Daniel 9:27. "And he shall confirm the covenant with many for one week: and in the midst of the week he shall cause the sacrifice and the oblation to cease, and for the overspreading of abominations he shall make it desolate, even until the consummation, and that determined shall be poured upon the desolate." So "in the midst of the week" refers to the middle of the 7-year Tribulation (or week). 

We learn that half of this 70th week equals 3 1/2 years in  Daniel 12:11, "And from the time that the daily sacrifice shall be taken away, and the abomination that maketh desolate set up, there shall be a thousand two hundred and ninety days. Blessed is he that waiteth, and cometh to the thousand three hundred and five and thirty days." So from the Abomination that maketh Desolate (Matthew 24:14), until Jesus' Second Coming is 1,290 days (or 3 1/2 years). That must mean the full 70th week in Daniel's prophecy is 7 years long (3 1/2 + 3 1/2). 

When Christ returns there will be the Battle of Armageddon and the Judgment of the Nations, which is why the Bible says whoever makes it to the 1,335 days will be blessed, because they survived it all. This is what "But he that shall endure unto the end, the same shall be saved" means in Matthew 24:13 (and not that a person has to persevere in holiness to get to Heaven, which is what Calvinists heretically believe). During the Judgment of the Nations, the tares (unsaved) will be removed and burned, sent to Hell to await trial at the Great White Throne of Judgment (which happens 1,000 years later in Revelation 20:11-15).