The tests for THC aren’t measured that way.
They test for trace in the blood.
However, trace from THC can remain in the system for around 30 days. Longer if they test hair.
"Delta-9-THC peaks in the bloodstream very rapidly after ingestion, and then is fairly quickly absorbed into the fatty tissue of the body. Dr. Novak indicated that Delta-9-THC can fall below detectable levels as soon as two hours after the use of marihuana. "