That's not even remotely close to being correct. There are many variables involved: 1. method of detection - urine, blood, tissue (HPLC or mass spec of hair can show drug use going back as far as the hair growth, which can be years in people with very long hair). 2. target of testing - depending on which metabolite you're looking for, LSD has metabolites that remain in the CSF almost indefinitely at low concentrations (again, only HPLC/mass spec can detect that). 3. duration of use - many metabolites have reservoirs in fat or muscle, and will slowly leech out over time. One-time use of cannabis can be detected in the urine for several days, but habitual use can fill the reservoirs and be detectable for months (as you said). Most employers will settle for an antibody-based urine test, which really only detects either very recent use, or long-term habitual use. However, if someone wants to drop $10k such as on a court-ordered test, they can find surprising things.