the data says
I’ve always looked to Size’s Database for answers to questions like these. However flawed the data is, there are enough entries to make some educated guesses with reasonable confidence.
To answer this specific question, I filtered the entries from the start date to a total of less than 105 days for any single user. This dropped the set of data to only 527 people, but still a reasonable set. There are many unfortunate things that you lose by doing that, the biggest to me is that you lose the data for everyone that very infrequently recorded their gains (or lack thereof). You also have a more conservative guess in that most of the entries are significantly less than 3 months. In fact, the average is 58 days for this set.
The average gain in length (including some ridiculous outliers that claim to have lost over 6 inches over that time) is .488 inches with a median of .400 inches.
The average gain in girth (including the same outliners with over an inch lost in less than 3 months) is .241 inches with a median of .188 inches.
The biggest concern here are all of the men that entered an initial size in the database, but never a second, indicating they never gained and gave up. Anecdotally, I would say that this is mostly due to lack of consistency, interest or commitment, but certainly there are hard gainers that haven’t budged with significant effort. Those “ghosts” are the biggest specter in the back of my mind anytime I analyze this data.
From a less scientific perspective, I think it would be absolutely reasonable with a conservative, but consistent routine. Newbie gains are consistently in that ballpark, and while maybe a bit too much to hope for in girth, still mostly attainable. Trying to avoid the cliche, everyone is so different, that I would have a hard time advocating a goal to achieve certain gains by a set time, especially without knowing how your body responds to any PE. Give it a shot, but be consistent and don’t do anything to harm your unit.