I can only speak on my own experience, back when I did manual stretches, jelqing and pumping I had sessions, daily, lasting 2-3 hours in one sitting.
Actually got about 1 cm in length from doing that for around 2-3 months. In my experience I’ve always had to go hard and that extra mile to get gains in any direction.
I’m that «more is more» type a guy, tried the «less is more» approach for years without gaining anything at all. Only when I put my heart and soul into this, as in very long sessions on the daily did I finally start gaining again.
Now I’m going the extender route clocking in thus far approximately 7 hours a day. Sure there’s some days where I don’t hit 7 hours but then again there are other days where I hit 8-9 hours so it all adds up.
Will be extending until I reach 10 inches BPEL, not joking as I want those double digits real bad. So let’s just say I’ll be extending for a good while into the future.