How does a full preterist interpret the 'Thousand-years reign' in Revelation 20 A full preterist is someone who believes that all the prophecies in the Bible have been fulfilled.

Then in Revelation 20:4 (RSV-2CE):


  Then I saw thrones, and seated on them were those to whom judgement was committed. Also I saw the souls of those who had been beheaded for their testimony to Jesus and for the word of God, and who had not worshiped the beast or its image and had not received its mark on their foreheads or their hands. They came to life, and reigned with Christ a thousand years.


How does a full preterist interpret the Thousand-year reign of Christ?
I take a more scientific approach to this. I believe this is simply a period of exponential growth of Christianity.
In order to explain this, I need to establish the beginning and the end given the context. I like to start with the end, because I am weird like that.
Christianity had a reign of 1000 years before heaven and earth are made anew in Rev 21:1, where &quot;Heaven and earth&quot; is used poetically to mean &quot;everywhere&quot;. This speaks of complete desolation, a barren land, as is the case after war. We know this because of the previous chapter, where it talks about Gog and Magog ending the 1000 year reign and then getting fire from heaven thrown down on them. The Turks conquered Jerusalem in 1073 AD and were hit with fire from heaven.
SOURCES:

https://en.wikipedia.org/wiki/Atsiz_ibn_Uwaq
https://books.google.com/books?id=sIJlMsv8gVIC&amp;pg=PA219&amp;lpg=PA219#v=onepage&amp;q&amp;f=false

The prior chapters to Revelation 20 deal with a very specific instance, an event that was fulfilled with the destruction of the Temple in 70 AD. A preterist takes the stance that John was writing this prior to the Temple's destruction.
SOURCE:

https://www.bible-history.com/jerusalem/firstcenturyjerusalem_destruction_of_jerusalem_in_70_a_d_.html

However, Jerusalem isn't 100% ruled by Christians between 70 AD and 1073 AD. In fact, it was under Muslim rule for some time after the Byzantine empire. Thus, it isn't a political interpretation but one of religious interpretation.
The percentage of the Christian world population was on the rise during this time. Christianity absolutely exploded. The science I talk about in the beginning is one of social science and statistics.  Exponential growth always follows a pattern as it reaches maturaity, an S-Curve or Sigmoid curve. This is well known in business marketing as a company reaches market saturation. You have slow growth, it picks up speed, then slows back down, and reaches a plateau or crest as the market or population becomes saturated.

This is quite common in populations, because once a thing like religion spreads, it can only go so far - the maximum population. You can only convert the maximum of 100% of people. The threshold for Christianity is probably a lot lower than 100% as given by the analogies Jesus gives for choosing the narrow path. If it is greater than the widest path, it is therefore not the correct path.
The thousand years I interpret as a period of exponential growth as a % of the population. This growth actually looks more linear according to my sources, though these are estimates. You will notice, the exponential growth seems to stop around 1100 AD (closest to 1073 AD) and experiences a slowing down of growth  where it reaches a peal in present day of around 31.7%.

Here is the Google Doc version: https://drive.google.com/file/d/1mLY2VEJSzTe65XTHFT2DMrX5xJdOJjnp/view?usp=sharing
Thus, the 1000 year reign is one of exponential Christian growth between the Jerusalem's conquest in 70 AD in the Jewish-Roman war to 1073 AD when the Turks conquered Jerusalem. It's not exactly 1000 years, but I'm sure you could make a case for corresponding millennial events.
That puts us in the new heaven and earth age. God's systems are pretty much set up. Christianity continues to bless the world by association (salt of the earth) at 31.7% and the dead are continuously judged as Revelation 20 dictates. Some may even say that is the reason for the plethora of near-death experiences seeming a lot like the New Jerusalem. I'll leave that up to the parapsychologists.