Hi, Doc;
You will find no formal studies about pumping and its permanent or temporary results and will have to rely on annecdotal replies.
In my case I used pumping exclusively before I even learned what jelking is. During a period of 4 - 5 months, I gained exactly an inch in length. I then went on an almost two week trip - no pumping - and the inch was still there when I returned. Is that “pernanent”? Who knows?
My thinking about permanence is this: If one gained an inch from pumping over whatever period of time of work, would one really care if the only way to keep that inch were to go back to the cylinder once,say, a week?
After I learned other PE exercises, I combined them with pumping. My best additional gains (another 1 1/3 inches) came during a time when I was alternating about 100 jelks with short pump sessions, 5 minutes or so at moderate pressure: jelk, pump; jelk, pump; jelk, pump. But my advice is pay attention to others’ routines only in the beginning, as you learn, then go with what feels like it is working or will work for you.