Some of the other responses seem reasonable for average propagation.
However, I wouldn't be surprised to see short-term variations (on the
order of seconds to minutes) up to tens of dB between antennas placed as
close as a wavelength or less, due to multipath propagation. You've
undoubtedly seen this as the "picket fencing" you get when using a
mobile VHF or UHF rig.

Roy Lewallen, W7EL

Ken Bessler wrote:
Assuming 2 identically equipped stations operating
mid day on 40m SSB, how far apart would they have
to be to see differences in propagation from a station
say 500 miles away?

Remember - both stations are identical - same rig, same
antenna (Inverted V @ 40'), same local noise levels, same
radiation patterns.

What I'm trying to visualize is the way a given signal hits
a city and whether an instance where station A can hear
better than station B is a problem with station B or simply
propagation patterns of an inbound signal.

Hope that makes sense..... lol!
--
73 de Ken KGØWX - Flying Pigs #-1055
Elecraft K2 #4913