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400 grams, slightly less than 400 ml, which is almost the normal amount for blood donation (donors give approximately 400-450 ml), and it is considered normal to donate blood every 60 days.
Can you imagine how many men would now be walking around with anemia and/or dying during sex from blood loss?
Everything Rmax wrote is correct; the amount of blood depends on the amount of seminal fluid, and the amount of the latter depends on the intensity of arousal. Arousal depends on a person's "tastes," plus the length of abstinence.