Watchtower August 15th 1935 - "...the greatest of all tribulations mentioned in the Scriptures must come after the great multitude appears and must be that which is mentioned by Christ Jesus as “tribulation such as was not since the beginning of the world to this time.” That tribulation period began with the “day of Jehovah”, to wit, in 1914, when the war started against Satan’s organization in heaven, and reaches the great climax at the time of the battle of Armageddon, the great day of God Almighty."
110 years - and counting!
Other denominations claim the great tribulation is only going to last for 7 years.