First of all, nowhere in the passage does it indicate [Christ] actually seated on a throne.
I do not believe the "Christ was not there -
- and then He was" interpretation of the passage.
As for the time frame:
~ after the crucifixion of Christ
~ after [all of] 'us' (in the context) were redeemed "out of every kindred, and tongue, and people, and nation"
~ after [all of] 'us' have been resurrected - having "glorified bodies" - allowable to be present in heaven
~ the number of which is "ten thousand times ten thousand, and thousands of thousands"
~ at a time when "every creature which is in heaven, and on the earth, and under the earth, and such as are in the sea, and all that are in them" could be heard saying "Blessing, and honour, and glory, and power,
be unto him that sitteth upon the throne, and unto the Lamb for ever and ever"