From the product read mes for the Genesis # Starter Essentials:

Genesis 2012-08-16 (but I think that is late in effect, the date DS 4.5 was released rather than the original beta version) Genesis 2 2013-06-20 Genesis 3 2015-06-23 Genesis 8 2017-10-12 (I suspect that is for the addition of the male, I think the female was in June)

So, it was less than a year from the official release of Genesis to Genesis 2, almost exactly two years from Genesis 2 to Genesis 3, and a few months over two years from Genesis 3 to Genesis 8. Genesis 8 has so far had three years.

I don't think there were any changes to Genesis and its AutoFit clones after the initial updates as DS 4 itself was being developed.