As some have mentioned on this thread and elsewhere, ranges from 3.5 to 7 years have been proposed.
However, this is wild speculation.
The source of that speculation, Daniel and Revelation, are both prophetic books.
So, the meaning of the words hour, day, year, etc have relative meanings.
Jesus words in Matthew regarding those days being "cut short" should be balanced with Revelation's warning that "this calls for endurance on the part of the saints" balances the equation.
In other words, it will not be so long that all flesh is destoyed, but not so short that it's over in time period that would be easy to endure.
BA- Honest answer.