Once a steady state is achieved, serum concentrations remain relatively constant over the 72 hours that a
patch is worn. After removal, serum concentrations of
fentanyl decrease slowly due to absorption of residual drug concentrations in the skin. The average half-life during transdermal application is 17 hours. Not surprisingly, this can vary considerably among individual patients.
I think the question is not how much longer does this last? I would ask why are suffering?