The authorities are really struggling over how to monitor usage. The 28 day rule does seem daft. The ▵9-THC concentration rapidly decreases to 1-4 ng/mL within 3-4 hour after smoking and this seems to relate to lack of clinical symptoms that might cause incapacitation. However if you eat it, the drug is absorbed in the fat and the effective plasma half life for frequent users is 13 days. In medicine we consider a drug to be ineffective after five half lives which is 65 days, but this is an average and might be 90 days in 5% of the population. Nobody is suggesting one marijuana cupcake makes you high for 3 months sadly but it may give positive toxicology results even though in Canada the pilot would be legal!!!! The paper referred to by T28B measures THCCOOH which is an inactive metabolite.
The rule will be 28 days because as often happens the rules are made by the regulator and dont need any scientific basis nor common sense, but the forensic toxicologists and lawyers may well grow rich on the court cases....