"And when he had opened the seventh seal, there was silence in heaven about the space of half an hour." Revelation 8:1.
In understanding this, we first of all need to rule out the application of the year-day principle in this case. We don't need to try to calculate this as prophetic time. The reason is that it happens after 1844. Revelation 10:6 indicates that after the experience described in Revelation 10, there would be "time no longer." What does "time no longer" mean?
"This time, which the angel declares with a solemn oath, is not the end of this world's history, neither of probationary time, but of prophetic time, which should precede the advent of our Lord." 7BC 971.
" 'Time no longer' (Rev. 10:5, 6). This message announces the end of the prophetic periods." 2SM 108.
So there are no prophetic time prophecies after 1844. This means that the "half an hour" of Revelation 8:1, which takes place at the second coming of Jesus, is not prophetic time, but must be literal time. What, then, is it describing?
Sequentially, the opening of the seventh seal immediately follows the sixth. The events that unfold at the opening of the sixth seal are described in Revelation 6:12-17. Chapter 7 is parenthetical, a thematic interlude in the narrative. So in real time, chapter 8 verse 1 immediately follows chapter 6 verse 17. The silence in heaven (8:1) comes right after the question is asked, "Who shall be able to stand?" (6:17).
Fortunately, we have some inspired commentary on these verses. This passage is explained in both The Great Controversy and in Early Writings:
"Before His presence, 'all faces are turned into paleness;' upon the rejecters of God's mercy falls the terror of eternal despair. 'The heart melteth, and the knees smite together, . . . and the faces of them all gather blackness.' Jeremiah 30:6; Nahum 2:10. The righteous cry with trembling: 'Who shall be able to stand?' The angels' song is hushed, and there is a period of awful silence. Then the voice of Jesus is heard, saying: 'My grace is sufficient for you.' The faces of the righteous are lighted up, and joy fills every heart. And the angels strike a note higher and sing again as they draw still nearer to the earth." GC 641.
"Then all faces gathered paleness, and those that God had rejected gathered blackness. Then we all cried out, 'Who shall be able to stand? Is my robe spotless?' Then the angels ceased to sing, and there was some time of awful silence, when Jesus spoke: 'Those who have clean hands and pure hearts shall be able to stand; My grace is sufficient for you.' At this our faces lighted up, and joy filled every heart. And the angels struck a note higher and sang again, while the cloud drew still nearer the earth." EW 16.
Revelation 8:1 tells us that this period of silence and suspense will last about 30 minutes.