How Long
In Matthew 24:36-41 Jesus states that only God the Father knows. With that said, Jesus continues by saying in Matthew 24:42-44 that we should keep watch.
In 2 Timothy 3:1-5 we learn about the state of mankind during the last days.
In Matthew 24:4-8 Jesus informs us of the state of the earth before His coming.
The Fig Tree
Some Bible scholars consider the fig tree to be a symbol for Israel. During the last week of Jesus' life before He was crucified, Jesus cursed a specific fig tree on His way to Jerusalem from Bethany as it wasn't bearing any fruit. Mark 11:12-14.
The next day, as He and His disciples were heading back into Jerusalem after spending the night at Bethany, they saw that the fig tree had withered from the roots. Mark 11:20-21
A couple of days later when Jesus was at the temple in Jerusalem and the disciples asked him when some these events of the last days would take place, Jesus said, "Now learn this lesson from the fig tree. As soon as its twigs get tender and its leaves come out, you know that summer is near. Even so, when you see these things happening, you know that it is near, right at the door. Truly I tell you, this generation will certainly not pass away until all these things have happened. Heaven and earth will pass away, but my words will never pass away." Mark 13:28-31.
The Nation Born in a Day
Isaiah 66:7-8 prophesied of a nation being born in a day. On May 14, 1948, that's exactly what happened when David Ben-Gurion declared Israel an independent State after the British Mandate over Palestine officially ended and UN Resolution 181 passed in the General Assembly.
In AD 70, Jersualem fell to the Romans and the Jewish temple was destroyed after a revolt by the Jewish people which started a few years earlier in AD 66. Jesus had foretold of the temple's destruction in Mark 13:1-2. This could tie in with Jesus' cursing of the fig tree that wasn't bearing fruit.
Israels rebirth in 1948 is considered to be symbolised by the fig tree once again getting twigs and coming into leaf.
The Generation
Jesus mentioned that this generation would not pass away until these thangs had taken place. The question is how long is a generation and to which generation was Jesus referring? Some say it was the current generation of people, others a future generation. Some lean on Psalm 90:10 to say that a generation is 70 years. They add 70 years to 1948, subtract the 7 year tribulation period and when May 14, 2011 came and went and they're still on earth, they had to amend ther estimate. Leaning on the same verse in Psalms, they say a generation should have been 80 years, so when May 14, 2021 came and went and they're still on earth, they again had to amend their estimate.
Jesus was using the fig tree as a symbol of Israel when he was talking about the generation. It seems more reasonable to consider that Jesus was talking about the generation of leaves on the fig tree. Each leaf could represent a Jewish person alive at the time Israel's rebirth who could bear witness to Israel once again becoming a nation. How long is the generation? Only God knows how long someone will live. The oldest poeple alive today are just under 120 years. Using that timeframe, May 14, 2068 would be in the upper range of dates for Jesus' second coming. If they need to be at least 20 years old to bear witness, that date would be May 14, 2048 for the fulfillment of end time events. Subtract the seven year tribulation period and the rapture could occur before 2041. Again, only God really knows and any date that anyone estimates is purely speculative.