Getting it cleared from your body will depend on your percentage of body fat, your metabolic rate,
and the ability of your liver to break down the
fentanyl. I would imagine that fentanyl could be found in your body for a month or so after stopping it, but the last couple weeks it would be in very low concentrations-- too low to measure by the standard urine tests out there, and too low to have much of an affect on you.
OK... hallmark of addiction...