When you stop the drug your nasal membranes will go back to the way they were prior to the use of phenylepherine. This will be back to baseline; not necessarily back to normal. Whether your nasal condition will require surgery will depend on what is causing it and the response, for example, to nasal steroid therapy.
Surgery is necessary for correction of a deviated nasal septum. Nasal polyps, on the other hand, frequently respond to steroid therapy. In the past surgery was thought to be necessary. You may want to try doing a nasal wash prior to your nasal steroid spray. Please click on this link to read more about this nasal wash treatment.
For some people, rebound happens after only 1-2 doses. I know it's miserable, but it should be better in a few days. Try Breathe Right Strips. In the future, try using less than the maximum dose, in other words use it at longer intervals, and the rebound won't be as bad. You can also try saline spray (Ayr)to clear your sinuses. Because this is not medicine, you can use it as much as you want. Do you have a sinus CT scheduled?