This falls under the catagory of: each person reacts differently
Just a quick synopsis of my journey...
Discovered MOS 10+ years ago and began a Bathmate/LengthMaster routine, starting stats (if I can recall correctly) were 6 3/8" in BPEL and 5 1/2" MSEG. Newby gains reached a plateau of almost 7" BPEL and 6 1/2" MSEG (girth was much easier for me to gain) and I stagnated there until I discovered the VacuVin mod for my Bathmate and I was able to attain pressures within the tube that I never thought possible. Since the BM does not have a pressure gauge, there's no way I could calculate the Hg's but it was easily 3 times what I could impart by hand.
This mod jump-started my gains and over the next few years I was able to reach 8" BPEL and almost 7" MSEG and I was also 'whole-package' pumping (not for everyone, LOL)...but again hit a plateau.
Since I was satisfied w/girth (actually it was a tad too much for symmetry), I purchased the MityVac w/the 2 1/2" cylinder...the thought was to limit any width expansion and impart as much force along the length axis. My current routine consists of about one hour in the cylinder at 90% to 100% max pressure (varying the amount every 15 minutes or so) and the Hg's are around 15. It took me a while to work-up to that amount of pressure, but it allowed me to break-through that dreaded plateau stage.
Current stats: 8 1/4" BPEL 6 3/4" MSEG (for a balanced look, I'd like to get to 8 3/4" in length, but PE is effing hard!)
I wrote this just to say everyone is different and finding your limits are very personal, (trust me, I've had blood in my Bathmate tube more times than I care to admit and the donut-effect is a real bitch to combat, but in the end I find it worth the effort)