I think that when Jesus asked the question in Luke 18:8 it was another way of letting us know that there would be a great apostasy just before the Second Coming. And it seems that Jesus put it in the form of a question as an invitation for us to think about it.
The Parable of the Widow and the Unjust Judge (Luke 18)
1 And he told them a parable, to the effect that they ought always to pray and not lose heart. 2 He said, “In a certain city there was a judge who neither feared God nor regarded man; 3 and there was a widow in that city who kept coming to him and saying, ‘Vindicate me against my adversary.’ 4 For a while he refused; but afterward he said to himself, ‘Though I neither fear God nor regard man, 5 yet because this widow bothers me, I will vindicate her, or she will wear me out by her continual coming.’” 6 And the Lord said, “Hear what the unrighteous judge says. 7 And will not God vindicate his elect, who cry to him day and night? Will he delay long over them? 8 I tell you, he will vindicate them speedily. Nevertheless, when the Son of man comes, will he find faith on earth?”
Cross-reference that with
The Seven Seals (Revelation 6)
9 When he opened the fifth seal, I saw under the altar the souls of those who had been slain for the word of God and for the witness they had borne; 10 they cried out with a loud voice, “O Sovereign Lord, holy and true, how long before thou wilt judge and avenge our blood on those who dwell upon the earth?” 11 Then they were each given a white robe and told to rest a little longer, until the number of their fellow servants and their brethren should be complete, who were to be killed as they themselves had been.
And cross-reference with
“And if those days had not been shortened, no human being would be saved; but for the sake of the elect those days will be shortened.”