a 1989 study of chronic users showed a maximum detection window of 25 days at a sensitivity of 20 ng/mL. Yet he points out that only one subject tested positive after 14 days, and it took an average of just 9.8 days before cannabinoid levels were no longer detectible. And while a 1984 study testing chronic users at a cutoff of 50 ng/mL showed a maximum of 40 days to get clean, 8 out of the 10 subjects needed only 13 days to show their first negative.
Source: How Long Does THC Stay in Your System? | Leafly