Watchtower articles used to refer to the original Greek, and then to various commentaries on a particular word or phrase.
How about this time?
Biblehub and 2 commentaries below:
http://biblehub.com/commentaries/matthew/24-34.htm
Barnes' Notes on the Bible
This generation ... - This age; this race of people. A generation is about 30 or 40 years. The destruction of Jerusalem took place about forty years after this was spoken. See the notes at Matthew 16:28.
Bengel's Gnomen
From the date of this prediction to the destruction of Jerusalem was a space of forty years, and from the true year of our Lord’s nativity to that event was a space of about seventy-five years. The Jews, however (as, for example, in Seder Olam), reckon seventy-five years as one generation, and the words, οὐ μὴ παρέλθῃ, “shall not pass away,” intimate that the greater part of that generation, but not the whole of it, should have passed away before all the events indicated should have come to pass. The prediction is true with respect to either the forty or the seventy-five years. So accurately did the Evangelist describe it many years before the event took place.