1/2” erect length gain in the first month or two is not unusual. Your flaccid size will probably also go up very noticably if you’re jelqing regularly.
I gained 1/2” length in about 5 weeks (almost exclusively jelqing), then no gains for about 3 months and I stopped PE. A couple of months later I came back and gained another 1/2” length in about a month’s time doing hard manual stretching. In the first year (9 months of PE) my total gains were 1” length, and .25” girth. I wasn’t to my goal size, but 7.5”x5.5” (BP) was satisfying enough that I didn’t have the motivation to do it non-stop. Four years later, doing periodic bouts of PE, my total gains are 1.75”x.6”
That’s my experience, but I have no idea what a “typical” gain would be. There are too many factors involved.