First the situation, then the question. Here's the situation:
An Earth-like planet had a civilization similar to ours, but when it emitted its first radio signals, a nearby alien observation post did an evolutionary reset of the planet by enveloping it in a shroud that wiped out the civilization via the old greenhouse treatment. Many years later, a human craft sets down on the planet and discovers the ruins of the civilization. This is where it gets tricky.
Since the planet is in the same general galactic neighbourhood as Earth, and since, in my story, there aren't many planets in the galaxy where advanced races ever develop, it feels too coincidental for the humans to find a planet where there are relatively recent ruins.
So I'd like to push the destruction as far back in time as I can while still leaving recognizable ruins for the humans to discover. I wish I could push it back millions of years, but there wouldn't be any ruins left and the humans aren't going to look for any esoteric traces deep in the ground.
So the question is:
Given that the civilization was technologically similar to ours mid-20th century, and given that the planet is being overheated by a planetary shroud, how many years back can I push the start of the destruction process and still have recognizable ruins when the humans arrive?
I know the pyramids and other stone structures have lasted thousands of years, but they're typically in arid lands unlike the wet and noxious world I envisage the ruins in my story will have to deal with. It doesn't seem like ruins would last very long in a greenhouse world, unfortunately. I'm guessing a few thousand years at most. Tell I'm wrong, please.
By the way, I realize the answer depends, in part, on how quickly the shroud overheats the planet, but all I can say to that is that the process needs to be done in a way that allows only simple life forms to ultimately survive it.