It seems pretty common from reading here, for someone to gain up to an inch within a relatively short period of time. I gained about and inch (in length) within 4 months or so, along with a little girth. Of that inch, I suspect that half was due to “regaining” some lost length due to an improvement in EQ.
Connective tissue has a certain degree of elasticity to it. People who start Pilate’s or yoga often find a significant increase in flexibility within the first few months, or weeks. After that, they can continue to gain flexibility, but the process is much slower. I am continuing to gain length, and now more girth, but at a much slower pace.
I think the mental attitude determines whether you will have sufficient patience, persistence and determination to continue beyond those early gains. If you start out running your first marathon thinking you probably won’t be able to complete it, you almost certainly will not. If you start out determined that you will finish it, you might.
As for the average total gain among those that stick with it for a couple of years or more, an inch and a half, or so, seems pretty common. If many members here are to be believed, gains greater than that are possible. I have heard some suggest that there is something physiologically unusual about those big gainers, and that seems possible. I suspect that they were just sufficiently driven, fanatical, etc. to keep going. A lot of guys who gain 1 1/2 inches in a couple of years are either going to meet their goal, decide that they have reached a point of diminishing returns that makes PE not time-effective, or will have a change in life circumstances that makes PE impossible or impractical.