2 Peter 3 draws on a statement from one of the Psalms to say "A day is like a Thousand years and a Thousand years like a day". The intent of this statement is merely a poetic idiom of God's Timelessness.
But it's popular from there to build a doctrine of the 7 days of Creation representing 7 Millennium of Human History.
It is interesting that this 7 Millenniums theory is expressed in The Talmud and other Rabbinic sources, and Christian supporters of these views keep pointing to the Rabbinic and Kabbalistic support of the idea to prop it up, since it lacks direct Biblical support. Still Christian support for the idea goes back to Ireaneus. But the Church Fathers who held this view believed Jesus first advent was around 5500 AM because they used the Septuagint, so they predicted about 500-5030 AD for the Millennium to start.
The core thing that makes it seem credible to Christians is the doctrine of The Millennium from Revelation 20., which becomes viewed as the Sabbath millennium.
One problem is the Revelation 20 Millennium is never Biblically defined as a period of rest or even of peace. 1 Corinthians 15, the only place outside Revelation that clearly addresses there being a period between the Second Coming and the full New Creation, says in verses 24-28.
"Then cometh the end, when he shall have delivered up the kingdom to God, even the Father; when he shall have put down all rule and all authority and power. For he must reign, till he hath put all enemies under his feet. The last enemy that shall be destroyed is death. For he hath put all things under his feet. But when he saith all things are put under him, it is manifest that he is excepted, which did put all things under him. And when all things shall be subdued unto him, then shall the Son also himself be subject unto him that put all things under him, that God may be all in all."It's defined sort of as a period of Conquest. Zechariah 14 also seems to show it as a time when Jesus is ruling, but people are obeying him out of fear not love. I've explained elsewhere how lots of passages people think are The Millennium are really the New Heaven and New Earth. Those include Isaiah 65, Ezekiel 40-48 and Psalm 48.
But the greater death nail to the theory is that in fact there is more then a Thousand years between the destruction of The Beast and the Descent of New Jerusalem..
Revelation 20 has many conditions, but only one is defined as ending "when the Thousand years are over", Satan being bound in The Abyss. The Beheaded Saints reign with Christ a thousand years, but it doesn't say their reign ends then. The "rest of the dead" are not raised till after the Thousand years, but it doesn't say that happens immediately either.
Satan has to deceive the Nations before the Gog and Magog invasion can happen. We have no idea how long he'll do that, it certainly could be very quickly, but maybe it could be a long time, years, centuries, maybe even another thousand years.
More important then that however is that I do agree with Chris White that the Gog and Magog invasion of Revelation 20 is the same as Ezekiel 38-39. In which case we know their dead bodies will be buried for 7 years before being raised for the White Throne Judgment. And the burying will take 7 months.
So I certainly think roughly 7 Thousand years is possible. But I would highly advise against making needing exactly 6000 years pivotal to your chronology. Though I have suggested one such theory in the past, but I had other reasons for that theory also, and now no longer support that chronology at all.
A stronger Biblical argument can be made for using the "Day like a Thousand years" idea with Hosea 5:15-6:2 for Two Thousand years from the Ascension. Which refers to YHWH returning to His place for two days and coming back on the third day when Israel acknowledges their offense. Hosea 5:15 is God talking. Chapter 6 records what Israel will pray in their repentance, 6:2 has the time reference in mind.
Which based on my 30 AD date for the Crucifixion can back up a 2030-2037 model with a 2033 Rapture.
Because Hosea's statement was a Prophecy, that works better then looking at 7 literal days that did literally happen and saying that tells how long history will be when no Bible passage directly tells us that.
But it could be Hosea is also talking about literal 24 hour days. Perhaps events that will transpire during the end times.
So I see Hosea as possibly implying about 2000 years from the Ascension to the Second Coming, but not necessarily exactly to the day or even exact year.
But I'm not gonna be dogmatic on that either, since I hate the Day=Year theory of historicists, I feel considering Day=Millennium definitive when the scriptural support for both is about equal is quite hypocritical. The Day=Year theory does have a precedent in Ezekiel 4, but that is Ezekiel doing something for days he's told represents years, it does not justify saying every reference to days in Daniel and Revelation is really years.
Many people arguing for the 7 Millenniums theory incorporate the Hosea reference into it. They argue Jesus was Crucified in the 4000th year from Adam, they provide no proper chronology to back that up, only asserting symbolic reasons it would make sense. Then cite Hosea to support 2000 years from the first advent to the second.
Another argument for 6000 years is trying to say the 120 years of Genesis 6 refers to 120 Jubilees and 50x120=6000. 1. That is clearly about 120 years before the Flood, no mention of Jubilees, a second application to human life spans/what a generation is possible but not solid. 2. A Jubilee is actually 49 years, the 50th year is the first year of the next cycle. 3. The Jubliee was given to Moses to be instituted by Joshua, it's not relevant to any pre-Mosaic history.
I lay out a possible opinion on when the year 6000 was here.
Genesis 5-6 don't have anything happen on exactly the year 1000, Genesis 11-12 has nothing happen on exactly the year 2000 though there is a rabbinic tradition based on the wrong 1948 AM birth-date for Abraham that he first became a Believer at 52. Ussher had the completion of The Temple in the year 3000 AM but if he was wrong on anything then that doesn't work. Ussher's Creation date could also be used to make the proposed 3 BC Birth of Jesus date 4000 AM, but I no longer consider that date valid.
So I see no reason to think the year 6000 was or will be important.