Well I was always a good gainer and I did 15 minutes of jelqing a session. Its a common misperception that the more work you do the more you will gain.
Maybe I should leave it to the anatomists here to give you a good explanation but not knowing something has never stopped me in the past.
What we are aiming for in excercises like jelqing is very small amount of tearing in the penis structure, something the body will replace with much the same tissue allowing us to strech more and gain more. If we stress the penis too much we end up with too much damage and then the body will both fight back by repairing the tissue with greater thickness and maybe replace good tissue with scar tissue (which though weaker is not ‘live’ tissue).
As we progress in PE the penis build up resistance, many people have found that overdoing it results in zero gains or maybe the same initial gain of lost length but stagnation soon after. Possibly this is because the penis builds up resistence more quickly.
Stretching or hanging is a different thing, it seems to work just as well if you do lots of it as opposed to a little of it. Maybe because it is more of a gradual thing, maybe because the penis is in a stretched state for longer.
Hope that helps.