With manuals, you don't necessarily have to work up to a certain number of reps or sets. Again, you go by feel. If you are comfortable at any given moment to increase the tension slightly, or to stretch a little bit longer, or if you feel like you could profit from doing an extra set at a particular angle, then do it.
You will feel the increasing intensity of the stretch with more reps. Some would describe it feeling good. But at some point, your body will warn you if you are about to go too far. The stretch will start feeling sharp, like it could possibly tear. At that point, you really have to pay attention. Do your reps, but with less force, until you feel like you have completely exhausted the tissues from all angles.
Regarding sets, you can increase them slowly if you want, but what is most important is fatigue in the target tissues. There were times where I was "supposed to do" three sets of each, but could only do 2 because my tissues were completely exhausted and my body was telling me to call it quits. You see? So I stopped.
So, I would do a quick warm up of manuals first thing in the morning, followed by extending, followed by your MAIN stretching routine for the day. For me, on average, my stretching routine lasted a good 30 to 40 minutes. After that, I would do 5-10 minutes sessions throughout the day (every 4 hours or so) to keep things loose.
The problem with pumping and girth work in general is that it sometimes interferes with your manual routine due to discomfort caused by edema. I do not want to tell you what to do, but if you are having this issue, it is probably best to do lighter girth work so that it doesn't affect your length work.
Maybe the lengthmaster solves this problem, I do not know. Someone will answer that.
But keep experimenting, and if you have any questions, we would be happy to help.