Doing some research the earliest sales point for FFVIII states that it sold 6 million which at the end of December 1999, meaning it sold 6 million in 11 months, 1 million of which it did in the US, and that same press release stated that FFVIII was the fastest selling FF, meaning it also sold 6 million faster than FFVII did within those 11 months from release.
https://web.archive.org/web/20080421065602/http://www.gamasutra.com/newswire/news/index19991206.htm
In 11 months FFXV had already reached around 6.6 million, making it the new fastest selling FF. XV also had the highest sales for any FF in the US, at about 1.8-1.9 million just for it's launch through to December 2016 period alone, which NPD cited was the highest selling FF in the US too.
It's much harder to find data during the late 90s and early 00s, but the next earliest point I count find was a financial report from March 2003, which for pretty much all the mainline FFs listed is well over 2 years after each of their releases.
www.jp.square-enix.com/ir/e/explanatory/download/0404-200402090000-01.pdf#page=27
By 2003 FFVII had sold 9.3m (6 years after release), FFVIII had sold 8.1m (4 years after release), FFIX had sold 5m (3 years after release), FFX had sold 5.8m (2 years after release) and FFX-2 had sold 2m just a few weeks after release.
For pre VII FFs by 2003 FFI had 1.9m, FFII had 1.2m, FFIII had 1.4m, FFIV had 2.1m, FFV had 2.6m and FFVI had 3.4m, and that's across all releases, and that's almost 10 years from release for VI and over a decade for all of the others.
As for FFXII, that only sold 6m total on PS2 by November 2009, and XIII didn't hit 6.6m until 2013, 3 years after each of those had released.
https://web.archive.org/web/20150505005802/http://gendai.ismedia.jp/articles/print/54
https://web.archive.org/web/2013082...y-xiiis-creators-justify-a-third-game-in-a-se