PS1:
4 months (one territory) - March 31st, 1995 - 0.85 million
10 months - September 30th, 1995 - 1.77 million
16 months - March 31st, 1996 - 4.26 million
22 months - September 30th, 1996 - 8.07 million
28 months - March 31st, 1997 - 13.50 million
PS2:
1 month (one territory) - March 31st, 2000 - 1.41 million
7 months (one territory) - September 30th, 2000 - 3.52 million
13 months - March 31st, 2001 - 10.61 million
19 months - September 30th, 2001 - 19.58 million
22 months - December 30th, 2001 - 24.99 million
PS3:
1 month - December 31st, 2006 - 1.7 million
4 months - March 31st, 2007 - 3.5 million
7 months - June 30th, 2007 - 4.2 million
10 months - September 30th, 2007 - 5.5 million
13 months - December 31st, 2007 - 10.4 million
PS4:
1 day - November 16th, 2013 - 1 million
2 weeks - December 1st, 2013 - 2.1 million
1 month - December 28th, 2013 - 4.2 million
3 months - February 8th, 2014 - 5.3 million
There's much celebration to be had at Sony HQ right now....PS4 is outpacing PS1, PS2, and PS3 at this time.