Re: A question for all.
It depends on whether you mean to the casual observer (fly over) or to an alien science mission looking very close.
If you use the Pyramids and the Sphinx as a benchmark, My SWAG would be 20-50 thousand years for all the landmarks to totally disappear but most would be gone a whole lot sooner. After about 10,000 years it would be hard to find traces but not impossible.
I imagine as long as the earth is here we and our trappings would always show up in the strata.