I'd recommend not to store the solution for more than about a week. Always make only the amount you need in a forseeable time. The hexacyanoferrate(II) is light-sensitive and can be oxidized to iron (III) compounds, so a brown bottle is the least you can do.
Please note that Ferricyanide is the Fe-III complex (K3Fe(CN)6), so it won't be oxidized by O2.
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