Postmillennialism teaches the following points:
(A) "…postmillennialism teaches that the 'thousand years' of Revelation 20 occurs prior to the Second Coming. (p. 10); An essential doctrine of postmillennialism is that prior to the Second Coming, the messianic kingdom will grow until it has filled the whole earth." (p. 191)
(B) “As far as Paul knew, Christ could have returned in his lifetime.” (p. 194)
(C) "When the word 'thousand' is used in Scripture, it refers to a literal thousand or to an indefinite, but very large, number." (p. 209)
[Cited from Keith A. Mathison's Postmillennialism: An Eschatology of Hope, (Phillipsburg: P&R Publishing, 1999]
Point (A) is what distinguishes Postmillennialism & Amillennialism from the other end-time views: it is what gives "Postmillennialism"' its name.
Point (B) is accepted by virtually all Bible interpreters based upon 1 Thess 4:17 and like passages, LINK.
Point (C) is accepted by all but the few "Full" Preterists.
(A) and (B) oppose (C): If Paul thought the Millennium preceded the Second Coming (A) that could have occurred within his lifetime (B), there is no way he would believe the Millennium to last 1000 or more years (C).
(B) and (C) oppose (A): If Paul thought Jesus could have returned within his lifetime (B) and that the Millennium is 1000 or more years (C), there is no way he would have believed the Millennium precedes the Second Coming (A).
(C) and (A) oppose (B): If Paul thought a 1000+ year (C) Millennium preceded the Second Coming (A), there is no way he would have expected Jesus to return within his lifetime (B).
Simply put, (A), (B) and (C) cannot each be true (unless the Apostle Paul be discredited, which is unthinkable, or that he thought his natural lifetime might extend over 1000 years, which is unsupported by Scripture). The accepted truth of (B) and (C) reject the conjecture of (A). Postmillennialism & Amillennialism are self-refuting. Read more ...
"Full" Preterists hold (A) and (B) by denying (C), claiming the "1000 years" was 40 years or less.
Idealists hold (A) and (B) by denying (C), claiming the "1000 years" is a mystic number without historical import.
Premillennial Dispensationalists deny (A), hold on to (C) but downplay (B), claiming "coming soon" can mean anywhere between zero to 2000 years or beyond.
70-1070AD Millennium denies (A) while holding (B) and (C), claiming "soon" means "soon" and "1000 years" means "1000 years" and Saints' reign with Christ begins at their Resurrection at Christ's Return, 1 Thess 4:17 = Rev 20:4.