Donk: Yes, during my final growth spurt I observed on 2 or 3 occasions that I had grown about 1/2" within a 2 week period. I was measuring frequently and carefully. I do not specifically recall growing an inch in a single month. Then again, I was measuring on a much more frequent basis than monthly, so maybe I did and just didn't think of it that way.
What I can say with certainty is that my final growth spurt added about 3" in length (8"-11") in about a 6 month period. It started 2-3 months before my 16th birthday and ended about 3-4 months after my 17th birthday. Before it started, I had not observed any growth for almost a year. (And I was measuring alot.)
Based on my experience, I would say that very rapid growth spurts are obviously possible and I think 1 inch in a month is not out of the question. Also, it is possible for a penis to grow in sudden spurts that are separated from each other in time. Puberty works in mysterious ways. | 
