God has set a definite limit to everything and everyone.

Revelation 11:1-8
Two witnesses will prophesy for 1260 days. They are the two olive trees and the two lamp stands that stand before the Lord of the earth. If anyone tries to harm them, fire comes from their mouths and devours their enemies.
When they have finished their testimony, the beast that comes up from the Abyss will attack them, and overpower and kill them at great city which is figuratively called Sodom and Egypt.
Closely reflects Jewish apocalyptic writing ( a text from Qumran, for example, concerns the measurements of the new Jerusalem) and is particularly difficult.
42 months: Equal to 1260 days, and 'a time(one yer), times(two years) and half a time (6 months)'(12:4). This may be derived from the length of the Syrian King Antiochus Epiphnes' tyranny in Jerusalem, which began in 168BC, or from Israel's 42 encampments in the desert. But the precise length of the trial is less important than the face that God has set a definite limit to it.
Why?
Great city here as later in the book, stands for the city of rebel humanity.
Olive trees and lamp stands like the perfume and light to attract people in the dark come to them, come to the Lord.
God is the Lord of the time, everything and everyone has time in his life on the world. No one is eternal in the world.
How?
God has set a definite limit to everything and everyone.