To lose the knowledge of how to make stuff? About a generation. But to have the concepts involved become mythologized? That generally takes around 3 centuries.
HISTORY MUST BE CURVED, for there is a horizon in the affairs of
mankind. Beyond this horizon, events pass out of historical
consciousness and into myth. Accounts are shortened, complexities
sloughed off, analogous figures fused, traditions “abraded into
anecdotes.” Real people become culture heroes: archetypical beings
performing iconic deeds. (Vansina 1985)
In oral societies this horizon lies typically at eighty years; but
historical consciousness endures longer in literate societies, and the
horizon may fall as far back as three centuries. Arthur, a late 5th
cent. war leader, had become by the time of Charlemagne the subject of
an elaborate story cycle. Three centuries later, troubadours had done
the same to Charlemagne himself. History had slipped over the horizon
and become the stuff of legend.
In AD 778, a Basque war party ambushed the Carolingian rear guard (Annales regni francorum). Forty years later, Einhard, a minister of
Charlemagne, mentioned “Roland, prefect of the Breton Marches” among
those killed (“Hruodlandus Brittannici limitis praefectus,” Vita
karoli magni). But by 1098, Roland had become a “paladin” and the
central character, the Basques had become Saracens, and a magic horn
and tale of treachery had been added (La chanson de Roland). Compare
the parallel fate of a Hopi narrative regarding a Navajo ambush
(Vansina, pp. 19-20).
This suggests that 17th century history has for the bulk of the
population already become myth. Jamestown is reduced to “Pocahontas,”
and Massachusetts boils down to “the First Thanksgiving.” And the
story of how heliocentrism replaced geocentrism has become a Genesis
Myth, in which a culture-hero performs iconic deeds that affirm the
rightness of Our Modern World-view.
-- The Great Ptolemaic Smackdown
The author here is speaking of the "Galileo was persecuted for believing that the Earth goes around the Sun" myth, (the truth is far more complicated, and far more interesting!), but the point raised here has plenty of other applications.
For example, we've all heard the idea that Columbus proved that the world is round. Not only is this so wrong that it falls apart under the simplest possible examination--take a look at a globe sometime and see if you can explain how sailing from Spain west to the Caribbean and then back east to Spain will prove that--he actually wasn't trying to. Everyone knew the world was round already, sailors most of all! (The visual phenomenon of a ship disappearing at the horizon from the bottom up, exactly as it would when moving along a curved surface, has been known since ancient times.) What Columbus tried to prove is that the world was a lot smaller than everyone knew it was--and they were essentially right about the size of the world, BTW--such that if you set out west from Europe, you could reach Asia before running out of provisions. He was wrong about that, and if there hadn't been more unexpected land in the way, he and his crew would have all died at sea.
But in the early 19th century--about 300 years after his fateful voyage--the idea arose, promulgated by Washington Irving, that Columbus was some paragon of Reason, proving to the benighted people of his day that the world was not actually flat as everyone believed, and the myth has stuck around in the popular consciousness ever since.
Therefore, by long historical precedent, if your society reaches a state where knowledge of modern concepts turns into myth, it's likely to take about 300 years.