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. 1. 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? 2. Do Jehovah's Witnesses have any official beliefs on how long the Great Tribulation is expected to last?