The Tribulation
How long is the tribulation? Or even, what is the tribulation? The idea of seven years split into two 3.5-year periods comes primarily from Daniel 9. Revelations has several periods in days and months that are around the 3.5-year periods running simultaneously. You have two distinct things happening. The first is the saints being persecuted by the beast during this period, primarily because of the extreme persecution for not taking the mark. The second is the calamities of the trumpets and wraths, resulting in terrible natural disasters. I am sure these are merely the main disasters, and there are many more happening as well all over the earth. The first period is 1260; this is the period that the ten horns get to rule the beast from Rev 13. When the Antichrist rises, he brings in the mark of the beast. The persecutions do not necessarily start when the last period of 1260 days begins. The calamities and persecutions start after the Antichrist arrives because the first wrath says that sores are given to those who have received the mark. Getting the mark out will take time. Furthermore, it does not say exactly how long the calamities will last. The first calamity falls on those who have received the mark, and that must take some time. The Tribulations finish before the return of Jesus, and the period of 75 days from Daniel 12 would fit. Although, the period of 75 days maybe a gap between the two periods of three and a half years. The locust gets to torment men for five months. So, the minimum would be five months if all these events happened simultaneously. That would be one hell of a day. I do get the distinct impression that the first five trumpets and wraths are not spaced out but happen quickly, one after another. Then there is a gap, and the last two happen. I guess time will tell.