Revelation 9: 5: And to them it was given that they should not kill them, but that they should be tormented five months: and their torment was as the torment of a scorpion, when he striketh a man.
The demons loosed from the Bottomless Pit under the fifth trumpet were likened to desert locusts. The lifespan of a desert locust is five months, so these devils are loosed to torment those who have rejected the Word until God is finished with them.
The fifth trumpet was also the first woe (Revelation 9: 12) or World War One. The five months here represent the five years of war that ravaged the world, from 1914 to 1918.