"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. "
http://www.courts.state.nh.us/superior/superorders/cronin.htm.
That would hold true for non-regular users, however, a regular or semi regular user would show trace full term.
A heavy user will show high trace levels.
Police/Law Enforcement tests for intoxication test for trace.
Now, the Medical Examiner, working on the Room Temperature user, the tests get a bit more detailed.