Is there a universal term for the "little time" (micros chronos) of the serpent being unbound after the millennium until the Great white throne judgement?
that would correspond to revelation 20:7-9, prior to eternity
I don't know, Erik, but there are two places that might give an indication - but not a guarantee of how long.
1. The 7th king is forthcoming in Revelation 17:10. When he comes, he must continue a short space. What it boils down to is the 7th king will be killed and come back to life, as the the beast and rule for the 42 months in Revelation 13. The 42 months is the short space.
2. In Revelation 12, when Satan is cast down to earth, as God starts dismantling his kingdom, he is full of wrath knowing he has but a little time left - which is expressed as a time, times, half times - a little less than the beast, by a few days or weeks. So essentially the same.
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Of related note, the 7 years begin; the first 1260 days the two witnesses testify against the Antichrist, thought-to-be messiah by the Jews;
near the end of the 1260 days, the Antichrist reveals himself as being the man of sin and betrays the Jew, is killed for his sudden claim to be God, and brought back to life as the beast; the two witnesses battle with the beast until day 1260, he kills them and they ascend on day 1263.5; leaving the beast to run wild 42 months after they are gone;
With His two witnesses martyred, God says of enough of Satan's garbage, and the 7th trumpet blows, and God starts dismantling Satan's kingdom overshadowing the world, beginning with the war of Satan being cast down to earth, with a time, times, half times left - which end coinciding with the end of the 7 years, which Satan then will be imprisoned when Jesus returns. For the thousand years.