As has been pointed out in other threads, L-arginine doesn’t work for everybody. When it does work, it seems to work well. We are all different from the point of view of our chemical makeups. I’ve taken a lot of L-a over years and have not noticed any gradual diminishing of its effect. But again, we’re all different.
Starting at a base point of 5 - 6 contractions [slang for that is “ropes”], you are about in the average ballpark. But the width of your urethra also figures in: a wide one and you will shoot more volume more quickly than a guy whose channel is more narrow, but is moving the same amount of fluid. How do you account for that? Seems like volume measurement is the only way and we know from “science” that 2 - 5 ml (CCs) is pretty much the average for most men. Can that be increased? I think, yes. And it does follow that with ejaculate volume increase, orgasm lasts longer.
Can’t help you with a container for measuring but I’d wager that a fertility clinic (every good sized city has one) would have info for you since they measure semen volume regularly when assaying male fertility.
There is, btw, a correlation between high semen volume and lower fertility. That may or may not be important to some interested in your experiment. Ditto exceedingly thick semen which does not tend to liquify within a time frame of about 20 min.. The little guys just can’t wiggle out of the clots before they get exhausted.