Anytime between now and when the rapture actually takes place. Could happen in the next minute. All other rapture views have a restrictive window that it must happen. The post trib view is very restrictive. The pre-trib view is less restrictive, because the window between now and the beginning of the 70th week is wider, but narrowing fast. The pre-wrath view mandates that the church be here during the first half of the 7 years. The mid-trib view mandates the rapture must take place in the middle part of the seven years, and that the church must be here for most of the first half..
Revelation 11:15 is not at the end of the end of the great tribulation, but near the beginning. The seventh angel sounds right after the two witnesses are killed on day 1260. Which begins a process of God taking the kingdoms of this world from Satan's domain. The first action will be to cast Satan and his angels down to earth - Babylon has fallen has fallen. God's judgement on the earth begins
Revelation 11:16-19, to understand those verses and the scene, you have to go back to chapter 4:1-4 when John is caught up to heaven and the twenty four elders are present, hearing what John is hearing as Jesus reveals the end times events beginning in Chapter 6. John's being caught up is part of the time of judging the dead which has three phases: the resurrection of the dead in Christ/the rapture; the resurrection of the martyr tribulation saints; the great white throne judgement at the end of the thousand years.
Obviously, the events of chapter 11 are not taking place at the end of the thousand years. So the reaction of the 24 elders is in respect to all three phases. You also have to go back to the beginning of Chapter 10, to understand that the 24 elders reaction, is after John receives the little book to eat.
Which, in chapter 10, it says in the days of the 7 angel's voice, the mystery of God would be revealed -
Revelation 10:7 But in the days of the voice of the seventh angel, when he shall begin to sound, the mystery of God should be finished, as he hath declared to his servants the prophets.
So it is talking about the end times, since the prophets declared those, including the 70th week in Daniel 9. Chapter 10 and 11 covers the 70th week.
I might also add, do you see anything in Chapter 6 as Jesus removes the seals - of Satan being cast down, or the mark of the beast, or worshiping the beast for 42 months? No. Those are in Chapter 12 and 13.... after the 24 elders have seen the 7th angel in Revelation 10 voicing and in Revelation 11 sounding. So you have to put yourself in John's shoes and the 24 elders being there in heaven as all things are being revealed for the first time.