I find that I build up a resistance to edema. While I full package pump, I stay around -4 hg. When I do -5 or anything more I get red dots, and unless I take a week off they continue even if I do not exceed -4 hg. If you soldier on though red dots without letting them heal they only get worse and I have read can lead to permanent discoloration. I avoid them by pumping at low pressure. Time does not seem to be a factor in the development of red dots for me, but can be a factor in edema build up.
Edema used to happen quickly, but as I have practiced pumping and slowly increased set times I can pump longer and longer without it developing in my shaft or testicles. I recently went from a 3.5 to 4 inch tube and have been finding I was getting a lot of edema in the mons pubis which expands more in the bigger tube even though I use the same seal for both cylinders. This seems to be less of a problem as I adjust to the new tube. I have also cut down on my pumping somewhat. I had been pushing and doing 2 sets of 45 minutes, but found my EQ to be waning, so I have gone to 1 set of up to an hour. I have also been experimenting with wet pumping, another variable that has been reported to reduce red dots and edema in some.
Stay low on the pressure and slowly increase your time by adding a few minutes a day until you hit a wall with either more edema than you like, or notice poor EQ. Back off from there until you get used to it, then you can slowly try slowly increasing your times again.
Pumping is an art that requires patience, persistence and lots of time.
I seem to get the best results with low and long sessions, and if your looking to grow your package and keeping a natural look, this seems to be the way to go. I am at close to a year of practice and only now feel like I know what I am doing, and only now feel like I am on the precipice of making permanent gains.