not sure but look at this powerpoint link for the army - slide 4 about thc levels indicate it depends on when smoked & what's done to flush body etc
http://www.google.co.uk/url?q=http://www.acsap.army.mil/programs/UPLCertOnline2/Track3/Nanogram.ppt&sa=U&ei=96GSUfrrEai54ASx44CADA&ved=0CCkQFjAE&usg=AFQjCNGkMZO7P-wuPhfLBzEGZaFKO0WeRw