The first thing is to define "extreme pumping". Without definitions on the part of the person asking questions, I am going to have to make assumptions or guesses so please pardon me on that part.
Pumping in general is different for everyone. The fluid being drawn into the tissues of the penis to create the expansion is in fact lymphatic fluid which is basically a percentage of the water making up your body. In the average individual who has never pumped these pathways are very small and drawing this fluid into this one area will because of this take a great deal of time. To get noticeable gain in size for a first timer will take anywhere from 45 to 90 minutes. As time goes on, the pathways begin to enlarge. Enlargement of these pathways begins to take place and is a prerequisite for permanent penis enlargement. You will not get the permanent enlargement until the pathways reach sufficient size to bring in enough volume to accomplish the stretching that is needed.
By extreme, I am going to assume that you mean pumping the penis to a point where it is stretched and grotesque in appearance. Again, this is an assumption because I do not have an exact definition.
What most do not understand is that there are many motiviations for pumping and the aspects that go with it. In order to get a permanent stretch, unfortunately this is a prerequisite. Some individuals find the great enlargement they experience to be erotic in and of itself and the visual eroticism of their own penis filling a tube much larger than it would normally be when erect does have an erotic quality to it. To some individuals, this erotic fantasy is what they seek more than any permanent enlargement from the act or labor they invest. The feeling when the penis is removed from the vacuum tube is completely different, and this in and of itself for some also has some erotic aspects. The imagination fills in what the physical cannot at least for a time.
No matter what the motivation, erotic and or permanent enlargement as time progresses, the pathways admitting the lymphatic fluid enlarge. In some, this can take place very quickly, in others this may take six months or more. Patience is a virtue and in all this will eventually happen if the rules of pumping are followed carefully. The pumper who starts out with a tube considerably larger in diameter than his erect penis is more than likely an "erotic pumper" meaning that this individual really enjoys the erotic aspects. The enlargement seeking pumper will start with a tube of a smaller diameter and he will pump in stages in a very deliberate and controlled manner. This pumper wants a great deal of tension (supplied by the vacuum) applied to the ligament structures and penile web. It is through this tension that stretching of the ligament structures will eventually take place, at least on a percentage of individuals.
The erotic pumper, using his larger vacuum tube on the other hand, wants to create an illusion of size maybe for himself and on occasion for others. Those who get together in "pump gatherings" usually feed on the more erotic aspects.
Usually figure that to get a penis to a really enormous size safely, the experienced long-term pumper with well developed pathways will have invested at least 2 hours in that project. Again the investment and total pumping experience will depend on the desired end result.
I hope that I have answered your question. Based on what I have to work with right now, this is probably the best that I can do at this time......Hope this helps answer your question.