How long for the epididymis to fill up fully?
The epididymis is never completely emptied. It is always being topped up = sperm output is geared to how frequently you ejaculate. The delay time between ejaculations is actually determined by prolactin levels rather than the ability of the prostate to refill. It is the refractory delay built into your brain system rather than the physical limits of the seminal vesicles that determines how many times you can c um.
The epididymis is full all the time. It is full of sperm in various stages of maturity. Semen only consists of 2% sperm cells, a tiny amount. So only a couple drops of your semen is sperm cells, by volume, and you will always have lots of sperm to ejaculate. What takes time depending on the guy is recharging and producing seminal fluids from the prostate and other glands. In the prostate sperm is mixed with the seminal fluid for ejaculation. You are shooting 98% semen when you ejaculate; reason you shoot so powerfully is you have held off ejaculating so your sexual urge is much stronger. Has nothing to do with sperm. And unless you are trying to father a child, frankly irrelevant whether you have a full load of sperm in your semen or not if you are just shooting off for personal pleasure. Most guys can ejaculate 3-5 times a day if they try; the prostate will produce seminal fluid on demand for the most part but still takes 30 minutes or so to refill. Not sure why you'd hold off 3 days to enjoy yourself, just do it when really horny and avoid porn when doing the deed, focus on your own sensations for max enjoyment.