What characteristics does the Great Tribulation have in JW eschatology? I had asked this question regarding LDS eschatology and thought that I ask the same one from Jehovah's Witnesses point of view. I can see however that there's this other question where the answer touches on some of the points, so I want to take my question on a slightly different route.


Do Jehovah's Witnesses have a doctrinal opinion on why Great
Tribulation is called "Great Tribulation", i.e. what makes it a
Great Tribulation? Are the reasons spiritual, economical, or
something else?
Do Jehovah's Witnesses have any official beliefs on how long the
Great Tribulation is expected to last?
Forgive me if I am unable to specifically answer your first question because I don’t know what you mean by “spiritual, economical, or something else.”  What I have done is research a Watchtower article dealing with the issue of the Great Tribulation and the events leading up to Armageddon:
Watchtower 15 July 2015: “Your Deliverance Is Getting Near”!   http://wol.jw.org/en/wol/d/r1/lp-e/2015523?q=deliverance&amp;p=par

14 What will happen after Gog of Magog starts the attack on God’s people? Both Matthew and Mark record the same event: “[The Son of man] will send out the angels and will gather his chosen ones together from the four winds, from earth’s extremity to heaven’s extremity.” (Mark 13:27; Matt. 24:31) This gathering work does not refer to the initial ingathering of anointed ones; nor does it refer to the final sealing of the remaining anointed ones. (Matt. 13:37, 38) That sealing happens before the outbreak of the great tribulation. (Rev. 7:1-4) So, what is this gathering work that Jesus mentions? It is the time when the remaining ones of the 144,000 will receive their heavenly reward. (1 Thess. 4:15-17; Rev. 14:1) This event will take place at some point after the beginning of the attack by Gog of Magog. (Ezek. 38:11)
16 Once all the 144,000 are in heaven, the final preparations for the marriage of the Lamb can begin. (Rev. 19:9) But something else will happen before that joyous event. Remember, shortly before the remaining ones of the 144,000 are taken to heaven, Gog will attack God’s people. (Ezek. 38:16)...  Together with his 144,000 co-rulers in heaven, Jesus will come to the rescue of God’s people here on earth.

As for any timeline, there is no indication in the following publication as to how long the Great Tribulation will last.  No dates are given, just this breakdown of events leading up to the Great Tribulation, which will culminate in the battle of Armageddon when God’s enemies are destroyed:
God’s Kingdom Rules (2014) Chapter 21 pp 222-230 – Timeline of Events pp 224-225:

”Before the Kingdom comes to put an end to its enemies, several significant events will take place.”

The precursor of the Great Tribulation will be a proclamation of peace and security.  This is to be followed by the final sealing of the remnant of anointed Christians.
The Great Tribulation begins with an attack on religion by the Wild Beast of Revelation (which is the United Nations).
Events leading up to Armageddon include celestial phenomena,  pronouncement of judgment on the enemies of God’s Kingdom by the Son of Man [Jesus], the all-out attack of Gog of Magog [Satan] on Jehovah’s people and then the gathering of the anointed ones.
These things happen before Armageddon, at which point the Great Tribulation ends.  The grand finale of the Great Tribulation is the execution of judgment at Armageddon.
Chapter 21 concludes with the following advice on how to prepare for and survive the Great Tribulation:

Survival will depend on obedience.   Such instructions come to us through the congregation arrangement.  Reference to Isaiah’s Prophecy – Light for All Mankind 1, pages 282-283.