FWIW, it took me 100+ hours.   Doing this right is a non-trivial undertaking
(in modern times, there are an astonishing number of changes per release).
That said, it is rewarding work that makes a difference.


[David Murray]
I can tell you that 3.4 took me approximately 67 hours according to my

time log.  That was going through the list prepared by Serhiy, and going
through pretty much all of the NEWS entries but not the commit log.  I'm
a precisionist, so I suspect someone less...ocd...about the details
could do it a bit faster, perhaps at the cost of some small amount of
accuracy :)