I don't know how long it takes to build up a dependence and/or tolerance, but as far as what withdrawal symptoms and how long they last, google gives me this;
Symptoms associated with fentanyl withdrawal may include any of the following symptoms:
• anxiety
• muscle aches
• back pain
• joint pain
• diarrhea
• rapid heartbeat
• rapid breathing
• fever
• tearing from your eyes
• runny nose
• sneezing fits
• chills
• sweating
• yawning
• anorexia
• nausea
• vomiting
• a feeling of your hair standing on end
• nervousness
• irritability
• shivering
• stomach cramps
• insomnia
• weakness
_
The first week is apparently the worst, but then it tapers off rather quickly at first, with some small withdrawal symptoms lingering a good long while.
Since it's only been two weeks of use, I wouldn't expect the worst withdrawal symptoms but then again, I do not know how quickly a dependence develops or how heavily you've been using it.
I truly wish you the best of luck in quitting.