Now learn the parable from the fig tree: when its branch has already become tender and puts forth its leaves, you know that summer is near; so, you too, when you see all these things, recognize that He is near, right at the door. Truly I say to you, this generation will not pass away until all these things take place. Heaven and earth will pass away, but My words will not pass away.
But of that day and hour no one knows, not even the angels of heaven, nor the Son, but the Father alone. For the coming of the Son of Man will be just like the days of Noah. (NASB)
The fig tree in this parable is often thought to be the nation of Israel. Interpreted, this passage above means that when you see the non-existent nation of Israel founded again, know that there will only be one generation and Jesus shall return.
Based on this above passage for centuries people have been predicting that Israel would return as a nation, and contrary to their scoffers have been proven right.
Israel began in to exist in its present day form when in 1917 the Balfour Declaration was made that committed the British Government to establishing a home for the Jewish people in Palestine. In 1947, the General Assembly of the United Nations adopted a resolution to create two separate states in Palestine, one Arab and one Jewish. Israel was to be given independence. This occurred in October of 1948.
In the light of what Jesus said and Israel being recognized as the homeland of the Jews, it appears that Jesus will be returning soon. As for the exact time Jesus will be returning, predicting the exact day is impossible. However, we have a clue as to when the year might be because Jesus said that it shall be as in the days of Noah.
A generation can be anything from 10yrs to 120yrs. Much depends on what is being talked about. A generation can be a lifetime or a predetermined number of years. In the case of Noah, the generation that is referred to is 120yrs.
What this means in the light of what Jesus said about his return is that he will return around 120yrs after the declaration that the Jewish homeland should be created.
This means that Jesus should be returning somewhere around AD2037. Frank Tipler has Jesus returning no later than AD2057 based on information other than interpreting what Jesus may have meant in this parable.
There are other prophecies that have to be fulfilled before Jesus returns, such as the restoration of the temple and the gospel being preached to every tribe on Earth.