Depending on the severity of the surgery, and FFS is severe, it can take 18 months or more for full sensation returns. And there is a fair possibility that is may not return completely or at all.
It has been over two years since my FFS and I still have some numb or light sensory areas on my scalp, face and chest. According to the doctor (Zukowski), I may never get complete feeling back. Dr McGinn, who also had FFS has virtually no sensation on her scalp seven years after her surgery.
It's been over 18 months since my SRS and I have pretty complete sensation there, though I was numb on the outer tissues for quite some time.
-Sandy