I find it’s always good to go back to your basic science to make predictions and theories.
Deformation: Intensity, Method and Recovery guidelines
(Don’t take the numbers verbatim; Shiver was citing data for tendons and ligaments. Although the tunica is made of the same type of collagen fibers as tendons and ligaments, the tunica’s structure is very different. In a tendon or ligament, the fibers line up together to give each other strength so the tendon or ligament can hold a large force. But the tunica is not designed for one dimensional loading like ligaments and tendons are, it can’t hold such large forces without taking some damage.)
The important thing to note is that the intensity of the stress determines how growth is triggered. Low intensity exercises cause no structural damage to the collagen structures; so to trigger growth you need to maintain the stress load for prolonged periods of time. High intensity exercises cause some structural damage to the collagen structures. Growth occurs from the body repairing the damage, so rest days are needed; this is also why hangers will use all-day compression wraps as well, you want the structure to repair itself while in the elongated state to get gains.
So what would we classify pumping and clamping as? Low intensity stress or high intensity stress? I’d say were probably dealing with low intensity here, for clamping this is simply because you’re relying on your own blood pressure to stretch the tunica. You aren’t going to be creating any really significant pressure on that alone. As for pumping, although you could go crazy and put your dick in a complete vacuum, I think it’s obvious that for safety reasons you’ll want to stay with low intensity, simply because you’ll injure yourself (major edema, broken blood vessels, etc.) long before you’ll ever cause any structural damage to the tunica’s collagen structure.
This corroborates with what clamping and pumping gainers have more or less said in this thread: you’ve got to put in the time. It’s something you’ll want to do daily, and for at least an hour; the more time you can give it the bigger your gains will be.