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#21
Some says (retailers most) that for an average use once a year is sufficient to drain and change the liquid, past average to mild/heavy use better change the liquid 2 times/year and for extreme using 3 times/year.
I'm between average and mild using so i'll do it twice a year to be sure having a fresh coolant not contaminated. Since the price of coolant is still attractive in 1liter (most needs 500ml) it can serves twice a year.
I've custom made loop, so for pre-filled ones i do not know the real cycle, i guess replacing is when the pump died.
Edited typo on my post above from "Useless" to "Unless", sorry for that little mistake.