Indeed. It's not difficult to pick any "project" - "repel the German
invasion", say, or "develop economic nuclear power" - and make it
nebulous enough to have a truly immense scope :-)

My back-of-the-envelope guess suggests around a billion man-hours of
effort - I don't have the time or inclination to dig up solid numbers
just now, but I can play around with it later if you want - for Apollo.
The Great Pyramid is vastly variable wrt assumptions, but maybe an order
of magnitude less?

Yes, I definitely wouldn't consider then interstate a "project" for
the purposes of this-- too many gradual goals: Half an interstate is
almost half as good as a whole interstate, and who can say when an
interstate is complete? And who was the charismatic leader who said
"We choose to build the interstate", etc. etc. etc.

The Great Pyramid strikes me as a really well-defined 'project'. The
Great Wall of China is trickier, because it was built by so many
different people over so many different time periods, and of course,
it had no clearly defined completion point.

WWII did strike me as probably being the 'activity' that had the
greatest number of man-hours, but it's not exactly a project per se.
I'm hard pressed to think of anything else that could even come close
to apollo. Human genome??? Some ancient building??

So how did you come to estimate apollo at 1 billion and great pyramid
at 1 million man-hours?