Gonna add a column of data...
No. Title Release date AGONIZING WAIT
1 Storm Front April 1, 2000
2 Fool Moon Jan. 1, 2001 9 mo
3 Grave Peril Sep. 1, 2001 8 mo
4 Summer Knight Sep. 3, 2002 12 mo
5 Death Masks Aug. 5, 2003 11 mo
6 Blood Rites Aug. 2, 2004 12 mo
7 Dead Beat May 3, 2005 9 mo
8 Proven Guilty May 2, 2006 12 mo
9 White Night April 3, 2007 11 mo
10 Small Favor April 1, 2008 12 mo
11 Turn Coat April 7, 2009 12 mo
12 Changes April 6, 2010 12 mo
13 Ghost Story July 26, 2011 15 mo
14 Cold Days Nov. 27, 2012 16 mo
15 Skin Game May 27, 2014 18 mo
16 Peace Talks
I suspect that the relatively-minor extra time for GS/CD/SG was for the complexity of laying down some pre-BAT material, and revising for consistency (per Beta feedback) with the ever-increasing canon... but maybe it was (in part or whole) dealing with some of the early stages of how things went so badly for him in this latest interregnum.
Also remember, his editor isn't a machine, either, and we don't know how much of a delay can be laid at the doors of after-Jim-hands-over-the-MS. people...
I doubt we'll ever get back to the days of 12-months-or-less-per-book, but I think 15-16 months is a reasonable hope.
ETA: Columnation... really sux. I swear, it looks fine in my compose/edit window!!!