2005: “June 6, 2005 — Apple today announced it expects to deliver over two million copies of Mac OS X version 10.4 “Tiger” by the end of this week, including copies sold at retail, copies delivered under maintenance agreements and copies bundled with Mac systems shipped, making Tiger Apple’s fastest selling OS release ever.”
2007: “October 30, 2007 — Apple today announced that it sold (or delivered in the case of maintenance agreements) over two million copies of Mac OS X Leopard since its release on Friday, far outpacing the first-weekend sales of Mac OS X Tiger, which was previously the most successful OS release in Apple’s history.”
So, number of days until 2 million copies (given an estimate of Thursday, June 9 2005 for the Tiger date):
Impressive.