Well, someone knows. Dust buries everything, my guess is that in dryier climates, such as Egypt where small pyraminds get buried, it's faster.

In South America, where the vegetation can get it, some of those ruins are 500 or 600 years old, and all but invisible.

In areas with a freeze-thaw cycle, larger objects can disintegrate. They get weakened, and a hundred-year quake gets 'em.

Disrepair is one thing. Buildings are "living" things, and need constant maintenance or they die, become useless. They don't go away, that takes dust, vegetation, freeze-thaw, and/or quakes.

Disappear is another.

In the long run, put your money on entropy.

Arf