Quite a long time, actually. If you have an old bottle, check for any sediment or STIB (slime in the bottle) and, if you find any, discard the ink responsibly. Also, if the ink smells musty or mouldy then also discard. Remember: ink is cheap, pens are not!
Also be aware that the fungicides used in modern inks are probably not as effective as those used some decades ago so please take care of your inks.
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