Some people, based on the following verse make a conclusion that Tribulation will b 7 years
Daniel 9:27 "And he will make a firm covenant with the many for one week, but in the middle of the week he will put a stop to sacrifice and grain offering.
However, this indicates the 7 years peace agreement, and no way it indicates that the tribulation must be 7 years cause antichrist will break the covenant in the mid of it.
Some people, based on the following concluded the Tribulation will be 3.5 years.
Rev 13 5 And there was given unto him a mouth speaking great things and blasphemies; and power was given unto him to continue forty and two months.
when the 5th angel sounded the trumpet of God then the beast of bottomless pit is released.
Rev 9:11 11 And they had a king over them, which is the angel of the bottomless pit, whose name in the Hebrew tongue is Abaddon, but in the Greek tongue hath his name Apollyon.
this beast of bottomless pit then killed the 2 witness of God after 3.5 years witnessing.
Rev 11 : 7 And when they shall have finished their testimony, the beast that ascendeth out of the bottomless pit shall make war against them, and shall overcome them, and kill them.
The 5th trumpet will sound and release the beast of bottomless pit after 3.5 years tribulation, this beast then kill the 2 Witnesses.
We know that after the 5th angel sounded the trumpets there still remain 2 trumpets to be blown and 7 vials of God to be poured beyond the 3.5 years of the Tribulation.
So......the tribulation is more than just 3.5 years but we cannot say it will be 7 years either.
Jesus bless u all,