First, what is the accepted meaning of the word “generation”?
A generation is "all of the people born and living at about the same time, regarded collectively." It can also be described as "the average period, generally considered to be about thirty years, during which children are born and grow up, become adults, and begin to have children of their own." A familial generation is a group of living beings constituting a single step in the line of descent from an ancestor. In developed nations the average familial generation length is in the high 20s and has even reached 30 years in some nations. https://en.wikipedia.org/wiki/Generation
This is what Jehovah’s Witnesses believed in 2010 regarding the length of the 1914 generation:
13... For example, consider our understanding of those who make up “this generation” mentioned by Jesus. (Read Matthew 24:32-34.) To what generation did Jesus refer? The article “Christ’s Presence—What Does It Mean to You?” explained that Jesus was referring, not to the wicked, but to his disciples, who were soon to be anointed with holy spirit.* Jesus’ anointed followers, both in the first century and in our day, would be the ones who would not only see the sign but also discern its meaning—that Jesus “is near at the doors.”
14 What does this explanation mean to us? Although we cannot measure the exact length of “this generation,” we do well to keep in mind several things about the word “generation”: It usually refers to people of varying ages whose lives overlap during a particular time period; it is not excessively long; and it has an end. (Ex. 1:6) How, then, are we to understand Jesus’ words about “this generation”? He evidently meant that the lives of the anointed who were on hand when the sign began to become evident in 1914 would overlap with the lives of other anointed ones who would see the start of the great tribulation. That generation had a beginning, and it surely will have an end. The fulfillment of the various features of the sign clearly indicates that the tribulation must be near. By maintaining your sense of urgency and keeping on the watch, you show that you are keeping up with advancing light and following the leadings of holy spirit.—Mark 13:37. (Emphasis mine)
Source: Watchtower 15 April 2010 pp7-11: Holy Spirit’s Role in the Outworking of Jehovah’s Purpose https://wol.jw.org/en/wol/d/r1/lp-e/2010282#h=17:220-18:787
In the 15 July 2013 Watchtower, the following admission was made: "In the past, we thought that the great tribulation began in 1914 when World War 1 started. We thought that Jehovah 'cut short' those days in 1918 when the war ended so that the remaining anointed ones on earth could preach the good news to all nations. Then we realised that a part of Jesus' prophecy about the last days has two fulfilments. So we needed to change the way we understood some parts of the prophecy."
This is what Jehovah’s Witnesses believed in 2014 regarding the length of the 1914 generation:
15 In his detailed prophecy about the conclusion of this system of things, Jesus said: “This generation will by no means pass away until all these things happen.” (Read Matthew 24:33-35.) We understand that in mentioning “this generation,” Jesus was referring to two groups of anointed Christians. The first group was on hand in 1914, and they readily discerned the sign of Christ’s presence in that year. Those who made up this group were not merely alive in 1914, but they were spirit-anointed as sons of God in or before that year.—Rom. 8:14-17.
16 The second group included in “this generation” are anointed contemporaries of the first group. They were not simply alive during the lifetime of those in the first group, but they were anointed with holy spirit during the time that those of the first group were still on earth. Thus, not every anointed person today is included in “this generation” of whom Jesus spoke. Today, those in this second group are themselves advancing in years. Yet, Jesus’ words at Matthew 24:34 give us confidence that at least some of “this generation will by no means pass away” before seeing the start of the great tribulation. This should add to our conviction that little time remains before the King of God’s Kingdom acts to destroy the wicked and usher in a righteous new world.—2 Pet. 3:13.
Source: 2014 Watchtower article, "'Let Your Kingdom Come'—But When?" https://wol.jw.org/en/wol/d/r1/lp-e/2014047#h=24
According to what Governing Body member David H. Splane said in the 2018 video clip (source link provided by 4castle), a generation is a group of contemporaries who have lived at the same time. For example, Joseph and all his brothers (Exodus 1:6) formed a generation. The generation Jesus spoke about (in Matthew 24:32-34) who would “see all these things” were the generation alive in 1914 who had the spiritual discernment to know that something invisible was occurring. Only the anointed remnant of Jehovah’s Witnesses had that spiritual discernment. Brother F.W. Franz, one of the anointed (and born in 1893) was the last of the FIRST group to discern what was going on in 1914. He died in 1992. Therefore, Mr Splane concludes, any of the remnant who were anointed before 1992 would be contemporaries of the first generation of anointed ones. The SECOND group of anointed ones are getting older, so we must be getting very close to the great tribulation which will culminate in “the end of this system of things.”
It is noteworthy that the original view that “the end of this system of things” would be witnessed by the generation alive in 1914 (those who understood what was going on in heaven) was jettisoned after it became apparent that almost all of them had died. That is when the new teaching about the meaning of “generation” was introduced, that it was now an “overlapping” generation. Apparently, any Witness who was anointed before Brother F.W. Franz died in 1992 has become part of that overlapping generation. The official view is that the 1914 generation is still going strong, because contemporaries of F.W. Franz are still alive. The length of the generation has been changed from meaning those alive in 1914 who understood what was going on (spiritually speaking) to meaning the generation that has overlapped the 1914 generation.
Second, Jehovah's Witnesses have not given any upper time limit for this “overlapping generation” although from quotes given above, that timeline cannot be stretched indefinitely: “That generation had a beginning, and it will surely have an end.” “...at least some of “this generation will by no means pass away” before seeing the start of the great tribulation.” Governing Body member David H. Splane was careful not to give any time limits, only to encourage everyone to strive harder because the time is near.
"The SECOND group of anointed ones are getting older, so we must be getting very close to the great tribulation which will culminate in “the end of this system of things.”
The following Wikipedia article gives useful information on when the current members of the Governing Body were appointed, along with dates of anointed members in 1971: https://en.wikipedia.org/wiki/Governing_Body_of_Jehovah%27s_Witnesses