Hello,
If the 2.5 inch diameter seems too large, I recommend maybe downsizing a tube. I use a 2 inch, a 2.5, a 2.75, and sometimes a 3.
The donut effect is due to the fact that the area beneath the head plumps up the largest, leading to that distorted look that many people associate with pumping and dislike so much. There isn't much you can do about it while pumping aside from maybe wrapping that area with something to keep it from pumping up out of scale with the rest.
As far as how long you should stay in the pump, breaks are always good, I say you know your body better than most people, so you can stay in the tube as long as you are comfortable and dont feel pain etc.
As far as permanent results, I've pumped for years, and maaaaaaaybe I've gotten fractions larger, but I doubt it. However I can say I can definitely pump up many many many times larger than I was able to when I first started. So theres that at least.
I've attached some pictures as well to show my unpumped state, and various cylinder sizes.