With 5 x 100mcg/hr patches, fentanyl levels in blood would peak at ~24hr's.. however, most non opioid tolerant users would overdose well before 24hrs. My experience with fentanyl patches/ duragesic is that it takes 8-12 hours for the fentanyl to be absorbed into the blood. I would definitely "feel it" by 8-10h, but really kicks in, in about 12hs & keeps building for another 10-12hrs. Stays stable for another 24hr's & then begins to decrease.