Dave made a lot of good points, so I will just put in my 2 cents of additional commentary....
Hydrocodone (Norco) and Fentanyl both have relatively short half-lives. When considering withdrawal severity and duration, the half-life of the substances is highly relevant; shorter half-life drugs producing shorter withdrawal (though sometimes more pronounced) while agents with longer half-lives can precipitate a less severe but more protracted withdrawal period (not to be confused with protracted withdrawal syndrome).
I would also inquire as to what reasoning the 5 month discontinuation entails. If there is any invasive procedure at that time, you will likely receive postoperative opioid therapy as well, meaning you may not experience withdrawal pre se from the fentanyl and Norco due to the cross tollerance with whatever opioid they use starting preoperatively, as well as perioperatively and postoperatively. Often fentanyl and morphine are popular in these settings, meperidine (Demerol) being much less common than previous practiced. Post-operative pain might be managed with any number of the opioids (Oxycodone, Hydrocodone, Hydromorphone, Morphine, Fentanyl, etc) and whatever it may turn out to be, it is likely that will be what you end up tapering off of permanently.
I whole heartedly agree with Dave regarding the power of suggestion and pre-anxiety of what might happen upon the discontinuation of the opioids. Your mind almost always fears the worst, with reality being less unpleasant than what might be expected (when there is apprehension and anxiety regarding the situation). The simple act of telling yourself and believing there isn't a need to worry will help more than almost anything else. The length of time tapering is really dependent on the substance but with those in question, I wouldn't (personally) taper quicker than 10 days or longer than 30 (I would probably aim for about 14-16).
Also as Shore said, opioid withdrawal in the worst case scenario is extremely unpleasant, but not life threatening or requiring emergent medical intervention. Severe acute alcohol withdrawal and Benzodiazepine Withdrawal Syndrome present as much more severe (potentially fatal) clinical situations and *are* medical emergencies (this is why benzos are never abruptly stopped). Your physician should know and assist you in the most effective discontinuation procedure. If you experience symptoms unpleasant enough, clonidine, diazepam (a benzodiazepine), and other medications are proven and have been documented to be effective in mitigation of the worst symptoms.
Best advice is to calm down, don't worry about it (especially so far off), and remember people all over the world do it all the time, every day without serious problems.
Good luck and always feel free to mention any concerns or questions