For those of us with SCI who can ejaculate, frequency is often inconsistent and may vary greatly over time. Semen is made of sperm from the testicles and fluid from the seminal vesicles and prostate. The prostate gland produces a thin, milky, alkaline fluid that accounts for most of the balance of the fluid. Together, the sperm, seminal fluid and prostatic fluid equal only about one teaspoon. Emission is controlled primarily by nerves exiting the spine at T11 and Tl2 and propulsion is primarily controlled by nerves exiting the lower spine at S2 to S4.
Ejaculation then occurs.
After ejaculation there is a so-called refractory period during which most men cannot respond to additional sexual stimulation. The length of this period increases with age. For men with SCI, the complex reflex responsible for ejaculation seems to become exhausted and needs much more time to recuperate. I've searched for a more complete answer to this problem, and this is the only explanation I've found. There are no nutritional supplements or special diets that will help you recharge any faster.