The ancient pyramids of Egypt are ridiculously cool.

Construction began over 4500 years ago, and these awesome structures, despite having been raided over the centuries and stripped of their beautiful white limestone exteriors, inspire millions.

Rebuilding them would be a massive project, although it probably could be done for a cool $5 billion or so--for just the Great Pyramid of Giza, that is.

They've lasted 5 millennia, and they will almost certainly endure after every human alive today is long dead. But the pyramids will not last forever.

As this Quora post notes:

The Rocky mountains in the US were formed over ~70 million years, ~70 million years ago. If geologic processes can turn an ocean into the Rockies in that time frame, it will make short work of the pyramids.

This other Quora post gives a more precise assessment:

Knowing that the pyramids 4000 years ago were made of polished limestone that would have glowed, we can say that it has eroded over time. I am not sure what the exact rate of limestone degradation is, and you could probably work it out by that rate, but by what I can see, around 2 layers of blocks and various bits have fallen from the sides of the larger pyramid. Many people have said that the pyramids would last 1 million years or even until the world ended, but I’d say around 10,000 to 100,000 years based on current observations.

What do you think? Assuming that humans do not intervene but the pyramids are just left to nature's tender mercies, how many years will it take, before the Great Pyramids are all eroded away into undetectability?

(Needless to say, this is one of those questions unlikely to be resolved on a timescale anyone cares about; it's just for fun.)