Why so? Started with lynnfield in mid 2009 and broadwell and skylake were released one after another last year that means 7 years of mainstream core i which satisfies the basic arithmetic.
I thought I'd replied to this. I wonder if it's appeared in another thread.
I thought It has released Jan of 2008, but it seems...
Lynnfield - - - - Sep 2009 - 3 months
Clarksdale - - - Jan 2010 - 12 months
Sandy Bridge - Jan 2011 - 16 months
Ivy Bridge - - - Apr 2012 - 14 months
Haswell - - - - June 2013 - 15 months
Broadwell - - - Sept 2014 - 11 months
Skylake - - - - Aug 2015
Not sure if Clarksdale counts technically as it was purely a die shrink and only used in dual core CPU's, so maybe 15 months for Lynnfield.
Also though Broadwell was much closer to Skylake.