The LAGEOS satellites, which are in an extremely stable orbit and made from dense materials that should maximize their stability, are expected to re-enter Earth's atmosphere in 8.4 million years.

The ones in high orbits would probably remain a very long time.

I wonder if NORAD would have detected it if a satellite remained from the previous civilization.

>I wonder if NORAD would have detected it if a satellite remained from the previous civilization.

We might've come pretty close:

https://en.wikipedia.org/wiki/Black_Knight_satellite_conspir...

Like, "in the order of a billion years" very long time?

Probably not.

The moon was formed in a massive impact 4.5 billion years ago which for a time, gave the Earth a ring. There would have been large, human-satellite-sized rocks at every orbital altitude.

None of these remain. We would expect the number remaining in orbit to decrease asymptotically over time, so if it has decreased to zero (or one, technically) then clearly satellites cannot remain in Earth orbit for many billions of years.

Even the orbit of the Moon has shifted significantly over billions of years.

Aside from orbital changes, things in space erode. A combination of meteoroid impacts, radiation, solar wind, and ablation will eventually obliterate most artefacts in space.

I don't know.