Originally Posted by Roger Hicks
Endless variables. Boil the water and let it cool (expels dissolved air) and store in half litre bottles: many months. Use well-aerated water out of the tap and store in big bottles: as soon as there's an air-space, it will last only a few weeks.
I vaguely recall that the oxidation is autocatalytic: in other words, a vicious spiral, where the oxidation products accelerate oxidation. But I can't remember. Marty (Freakscene) will know far better than I.
Roger is right about autocatalysis - it is best to prevent oxidation from _starting_ rather than thinking that you can arrest it once it has begun.
Water is infinitely variable - I am not sure how much boiling your mix water will help, because although it drives oxygen off temporarily, it does nothing to the ions dissolves in it that are most likely to start the oxidative reactions, and even a tiny air pocket will allow the water to re-equilibrate to ~2 mg/L dissolved oxygen pretty quickly.
I'd say for optimum storage, mix your D76 with deionised water, keep it in full, well-sealed bottles in the dark, or by replacing the air with a non-reactive gas (wine preserver works well) if you need to have a space, and keep careful notes on how it stays active in storage under your conditions. Keep careful track of your results.
I tend to mix a batch and use it within days, so I am not very helpful except in theory, unfortunately.