AC's 1st volume hit 1 million in print in 26 weeks approx. It debuted in Nov 2012 and by May 2013, Vol 1 was already at 1 million print.
https://mantan-web.jp/article/20130513dog00m200017000c.html
So, SxF is running to be 3rd quickest Vol 1 to hit 1 million then.
That's very fast indeed
But, if we are using print data, then there are probably more older manga that reached 1m in under a year (I haven't checked but I wouldn't be surprised if manga like Slam Dunk and GTO did it).
And if we use sell-through data only (so, Oricon weekly/yearly rankings since 2008), then I found that it took a little bit more time for AC to reach 1m.
My reasoning is the following:
It came out in November 2012, had 3 weeks to sell in the 2012 Oricon year during which it sold 261,509 units.
It then appeared in the 2013 ranking of the best-selling volumes. It apparently sold 756,451 more units. That's a total of 1,017,960 units in 55 weeks.
So, we can't precisely know when it reached 1m units. If we use the Shoseki rankings to know how much it sold in its 53rd/54th/55th week, we get the following:
Week 53: Ranked #143. #100 sold 9,300 units, #200 sold 4,900 units. AC's Volume 1 should be around 7,408 units.
Week 54: Ranked #147. #100 sold 7,900 units, #200 sold 4,550 units. AC's Volume 1 should be around 6,325 units.
Week 55: Ranked #194. #100 sold 6,500 units, #200 sold 4,000 units. AC's Volume 1 should be around 4,150 units.
For a total of 17,883 units, meaning that after 52 weeks, AC's first volume had sold about 1,000,077 units. Shoseki isn't Oricon, so we can't quite compare them, but it does seem to show that AC's first volume reached 1m units with its 52nd or 53rd week