Jesus talked about seasons and times, and as you all know said this generation will by no means pass away until all these things occur (Math 24: 34) a biblical generation is 40yrs, Noah built the ark the rains came after 40yrs. The Isrealites where in the desert for 40yrs, and I am sure other bible knowledge folk on here could think of other 40yr generations that only deal with God's people.
The great tribulation will be something mankind has never seen or experienced according to Jesus, so just maybe that generation starts at the on set of the great trib and lasts fourty years, but true to Christs' words, we dont' know the hour or the time of the great trib, which leads to armageddon, only those that remain faithful through the great trib survive armageddon.
I personally believe it is a spiritual 40yr desolation and when that started we don't know, but the WTS has certainly come to a heavy low the past 30yrs or so, and I don't believe they are yet judged, so this 40yr biblical generation could mean spiritual desolation of the WTS, until the presence of the Christ and thats when the final 144,000 are sealed and proven worthy of the kingdom. Just a thought.