The windows of time in which the fentanyl in Duragesic can be detected in your system will vary between the type of test you are administered. A rough estimate as to how long it can be found in your system is as follows: Urine: can be detected up to 3 days after its last use. Blood and plasma: up to 12 hours after its last use. Fentanyl’s major breakdown product norfentanyl is detectable in the blood for up to 10 hours. Saliva: It is unclear how long fentanyl is detectable in saliva. Hair: In general, 1.5 inches of hair growth shows the past 90 days of drug use.