I dunno, how long did it take for the Roman Empire to be at its peak to when it just became references in the history books of the nations that came after?
That’s why the question of “how do you define collapse” is important.
New York, Pennsylvania and Maryland might well hold on as “The United States of America” with DC as the capital and something based in our constitution for another thousand years, but if the rest of the country has broken off I’d call that a collapse.