How do mainstream Christians handle the Epistle of St. Barnabas' prophecy that Jesus will return in the next 50 years? In the Epistle of Barnabas, it says


  Barnabas 15:3 Of the Sabbath He speaketh in the beginning of the
  creation; And God made the works of His hands in six days, and He
  ended on the seventh day, and rested on it, and He hallowed it.
  
  Barnabas 15:4 Give heed, children, what this meaneth; He ended in six
  days. He meaneth this, that in six thousand years the Lord shall bring
  all things to an end; for the day with Him signifyeth a thousand
  years; and this He himself beareth me witness, saying; Behold, the day
  of the Lord shall be as a thousand years. Therefore, children, in six
  days, that is in six thousand years, everything shall come to an end.
  
  Barnabas 15:5 And He rested on the seventh day. this He meaneth; when
  His Son shall come, and shall abolish the time of the Lawless One, and
  shall judge the ungodly, and shall change the sun and the moon and the
  stars, then shall he truly rest on the seventh day.


And we know that the World (Human History) according to Bible is now almost 6000 years.
This means The Second Coming of Jesus will happen within 50 years from now.

According to  Saint Malachy Prophecy
 we can know that it has the same timeline.

How should we interpret these things?
The Epistle of Barnabas is not considered canon. Sticking purely to canon, the bible is very explicit that we do not know. The world could end tomorrow. Or this afternoon. Or 10,000 years from now:

Matt 25:13  


  Watch therefore, for you know neither the day nor the hour wherein the Son of man comes. 


Mark 13:32   


  No one knows about that day or hour, not even the angels in heaven, nor the Son, but only the Father. Be on guard! Be alert! You do not know when that time will come.


1 Thessalonians 5:2  


  for you know very well that the day of the Lord will come like a thief in the night.