I dont think we will ever have a definate answer because when we have a survey there are guys that have been at it for 6 months and guys that have been in it for 10 years, i would say it is a pretty safe bet that anybody who really pays attention to what works for them and stays the course can easily gain an inch in length and a 1/2 inch in girth in a year, some do not and will not like me saying so but then again for everyone that gains nothing some lucky prick gains an inch and 1/2 (prick pun)
I have gained just over an inch in length and 1/2 inch of girth in just under a year and 1/2.