When did the Lion of the tribe of Judah take the book from the One sitting on the throne? And he came and took the book out of the right hand of him that sat
  upon the throne (KJV, Rev. 5:7)


When did this moment happen? When did the Lion of the tribe of Judah (Jesus) take the book from the right hand of the One sitting on the throne (God the Father)? 

(Has this moment already happened in history? If yes, when? Or is it still to take place in the future? Or is it something that is the reality outside of the realm of time and, therefore, has its impact in various degrees and ways on all ages of human history?)

Anyway, how is this question traditionally answered in Christianity? (By "Christianity" I mean those people who address Jesus directly by His name in their prayers.)

If there are differences in traditions (and, therefore, in the ways this question is answered) between different branches of Christianity, then I am more interested in how this question is usually answered in main-stream Protestantism.
After Darby, most Protestants believe that Christ has NOT taken the scroll/book.  They see it as a future event.  Prior to Darby, Protestants taught that this was historical; it had already happened.  Here's why.

Revelation was written after Christ had been resurrected and ascended.  When John sees the book/scroll in the hand of the one on the throne, he weeps because no one was worthy.  The vision at that point was of the time prior to ascension.  In other words, John already knew, but was as if he had no knowledge of this.

One of the elders tells John not to worry.  There is the Lion of the tribe of Judah.  He prevailed.  Now we are at the time post ascension.  Christ has purchased and redeemed us out of every nation on earth.  John knows this in "real time".

The word "throne" means also "seat".

Throne:

"a throne, a seat of authority," is used of the "throne"
(a) of God, e.g., Hbr 4:16, "the throne of grace," i.e., from which grace proceeds; Hbr 8:1; 12:2; Rev 1:4; 3:21 (2nd part); 4:2 (twice); 5:1; frequently in Rev.; in Rev 20:12, in the best texts, "the throne" (some have Theos, "God," AV); cp. Rev 21:3; Mat 5:34; 23:22; Act 7:49;"
Vines Expository Dictionary of New Testament Words

This would be in the holy of holies.  So Christ's sacrifice was accepted.  He was resurrected.  He ascended.  He entered the throne room.  He took the scroll.  He opened it.

To back up a bit to get another time marker, John's vision of the throne "room" begins when John is called up into heaven.  He sees various things, including the seven-fold Spirit of God.


  And out of the throne proceeded lightnings and thunderings and voices: and there were seven lamps of fire burning before the throne, which are the seven Spirits of God.
  Rev. 4:5


The sequence after this is no one is found worthy.  John weeps.  Christ the Lamb slain appears.  When was this?


  And I beheld, and, lo, in the midst of the throne and of the four beasts, and in the midst of the elders, stood a Lamb as it had been slain, having seven horns and seven eyes, which are the seven Spirits of God sent forth into all the earth.
  Rev. 5:6


The point is that John first sees the Spirit of God at the throne, but then after ascension, the Spirit was sent forth (perfect tense) on Pentecost CE 30.


  And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance.
  Acts 2:4


That was Pentecost on a Sunday 10 days after Christ ascended.

So, John is viewing Christ as taking the scroll on Pentecost after the Spirit was poured out earlier that same day.