Depends on the molecule. Chelated iron won't oxidize until it is seperated from the acid. If it's pure iron, I would imagine it will begin rusting immediately unless kept in an oxygen free environment. A low pH solution will last a longer time, whereas a more neutral or basic solution will break apart the molecule and cause free iron to drop out of the solution.
Also, DPTA is more stable than EDTA iron unless the pH of the solution is very low. In powder form, with near zero humidity, either will last a very, very long time. When mixed with most tap water, it will begin to degrade quickly. I would expect you could get weeks out of distilled water without a major breakdown. I haven't tested for this as I don't have the proper equipment. My iron test appears to register all iron in the solution or suspension, regardless of what other molecules it is affiliated with.
Hydroponic solutions are usually kept below pH 6.5 to help keep iron and manganese in the solution. Once the solution gets into the neutral range, iron precipitates out quickly.