Nov 10, - More likely, you want to know how long THC stays in your urine because Urine drug tests for weed, then, are essentially trying to detect. Amphetamines: days in urine and around 12 hours in blood. Barbiturates: days in urine and days in blood. Benzodiazepines: weeks in urine and days in blood. Cannabis: days in urine and up to 2 weeks in blood.