Christ doesn't come back until ______? Every now and then you hear someone set a date for Christ’s return. 

Such dates are bound to fail –


  Matthew 24:36  "No one knows about that day or hour ... "


And when they do fail - 


  (Issac Newton) was wary of prophetic date-setting. Newton was worried
  that the failure of fallible human predictions based on divine
  prophecy would bring the Bible into disrepute. (Source)


To stop the doomsayers - 


  He estimated that the world would end no earlier than 2060. In
  predicting this he said "This I mention not to assert when the time of
  the end shall be, but to put a stop to the rash conjectures of
  fanciful men who are frequently predicting the time of the end, and by
  doing so bring the sacred prophesies into discredit as often as their
  predictions fail. (Source)


Now my question - 

What is the time or events before which Christ couldn't return no matter what? (to put a stop to the rash conjectures of fanciful men and satisfy my curiosity)
I believe the only possible answer to this question, that all Christians can agree with is:


  For you*, Christ cannot possibly return any sooner than right now.


All anyone can know is that Christ has not yet come, but that he will come in the future.  That may be in five seconds, or in 50,000 years. Biblical prophecies are not specific enough that it is possible to come to any consensus as to what specific historical events they might refer to.

The fact that people have been making specific predictions of the second coming for centuries should be the first clue that not everyone will agree there are still specific events that "must" occur before the second coming.

* Some believe that Christ's second coming is referring to a person's individual judgment at the time of death. In that view, the second coming has already occurred for everyone who has died, and is still in the future for everyone else.