How long to fill up balls completely?

No, your testicles don't get bigger if you don't have sex. The size of your scrotum doesn't change if you don't ejaculate for long periods of time.
Testicles do temporarily expand during sex because of increased blood flow to the genitals.


They return to their normal size after arousal subsides or after orgasm.

Why are my balls bigger after sex?

The testes do get bigger during sexual arousal and return to their previous size after arousal subsides or after orgasm, but that's only because of shifts in blood flow—the same reason as for the size changes of the penis.

So what ever method you are using to measure the size of your balls, try it after a cold shower.

 

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Way to throw cold water on a hot topic! Pun intended.

 

No, I think he meant when do they fill right up, grow to massive size and then burst violently, leading to multiple (and very sticky) casualties?

 

Balls Don't Actually Turn Blue — But They Can Double in Size
If your balls feel full and appear larger, it's usually because you're aroused. But if you're aroused and don't get any release via an orgasm, you may also experience an uncomfortable aching feeling in the testicles, known as "blue balls." However, despite the name, your testicles don't actually turn blue. At most, you might notice a slightly blue tint because there's more blood volume.

Blue balls, known medically as epididymal hypertension (EH), happen when the blood vessels to the penis and testicles expand to make room for increased blood flow, said Dr. Reitano. This is how the penis becomes erect for sex, and you may also notice the testicles almost double in size.

For those that are interested reading
https://www.health.com/condition/sexual-health/testicle-facts

 

After 9 days of denial the urge is at highest level, then the body gets used to chastity. Without stimulation my locked penis falls in sleeping beauty sleep mode. The amount is varying, longer term of chastity does not automatically lead to a big load

 

so how long does it take to abstain in order to get maximum sperm?