Solus.... I change my stored water out at the beginning of hurricane season each year (June for me), so mine is no more than a year old... I add the 2 milliliter of unscented Clorox for each of the 20 liter (5 gal containers) at the time they are filled, and not when used... I did not write down where I got the test strips, but found some with low part per million test range... around 3ppm is recommended to sufficiently kill the critters... I have a Sawyer .1 micron 5 gallon bucket system just in case
I would think that as long as the available chlorine is 2 to 3 ppm, it could be stored for a couple of years... my thinking is that the chlorine in the water will help inhibit any growth in the container to some extent
the 2ml comes from 5 gal x 8 drops = 40drops.... typically 20 drops per ml, so 2ml give me the 8 drop concentration