Arthur wrote:
snip
long after the crows have picked our bones clean.
have scientists speculated on how long it would be before avery man made
thing has ..in short, turned to dust?

It depends.
The apollo landers will be easily visible for millions of years.
Many structures, such as mines in rock, infilled with debris, will be
visible until the rocks they are in are destroyed.

So, billions of years.
Nuclear waste dumps will have anomolous isotope readings until then too.