I’m thinking that from previous PE Data site analysis, and from polls, that the average length gain is about 3/4” and the average girth gain is about 3/8”. The data is a bit old though. That’s just an average. The more consistent and dedicated ones are the ones who ‘tend’ to achieve more. Also, in general, from a poll it appears age possibly plays a role in how much one easily, moderately, or with difficulty gains.
The average gains aren’t more because many abandon, or almost abandon, dealing with PE after a while. I doubt most PEers stay consistently with it for as long as 2 to 3 years and that pulls the average down, no pun intended. I would think that at least a 1” length gain and .5” girth gain is probably about right amongst those that stay with it for that length of time with consistency. As marinera pointed out usually only the most seriously dedicated have a chance at achieving much more.