My experience exactly matches yours. About three years ago I decided
to see how long I could make chains last on my general purpose bike by
thorough cleaning (solvent baths) as often as I could bear it (every
2-3 weeks, about 300-400 miles).

Answer: the same as they lasted before (and since).

On the other hand I have one bike I only ride in fair weather in the
summer. A chain on this lasted me for 20 years (sorry, can't convert
to miles as this was pre-computer days - this is a low mileage bike,
estimate only about 500 miles per year).

I conclude that extending chain life is only possible in two ways:

1. By OBSESSIVE cleaning after each ride (though I have only hearsay
evidence for this).

2. Only riding in clean, dry conditions (evidence from personal
experience).

Now I am happy to get 2000 miles out of a chain (sometimes a bit more,
sometimes a bit less), safe in the knowledge that to get more requires
a degree of dedication to cleanliness that I just don't have (one
glance at my bikes confirms this).

I do not believe that the terrain you ride is a material factor.

Andrew Webster