In Matthew 24, Jesus spoke of the abomination of desolation immediately followed by great tribulation.

Mat 24:15  “Therefore when you see the ‘ABOMINATION OF DESOLATION,’ spoken of by Daniel the prophet, standing in the holy place”  (whoever reads, let him understand),
Mat 24:16  “then let those who are in Judea flee to the mountains.
Mat 24:17  Let him who is on the housetop not go down to take anything out of his house.
Mat 24:18  And let him who is in the field not go back to get his clothes.
Mat 24:19  But woe to those who are pregnant and to those who are nursing babies in those days!
Mat 24:20  And pray that your flight may not be in winter or on the Sabbath.
Mat 24:21  For then there will be great tribulation, such as has not been since the beginning of the world until this time, no, nor ever shall be.

From this we see that the appearance of the abomination of desolation is immediately followed by the great tribulation. 

The book of Daniel has more information about the abomination of desolation.

Dan 12:11  “And from the time that the daily sacrifice is taken away, and the abomination of desolation is set up, there shall be one thousand two hundred and ninety days.

Sometime leading up to Jesus’s return the daily sacrifices will be reinstituted.  From the time they are stopped, and the abomination of desolation is set up, to the time of Jesus’s return will be 1,290 days, which is about 3½ years.  This means that the abomination of desolation and the great tribulation both occur in the last 3½ years.

So the great tribulation lasts approximately 3½ years.