Once opened, distilled water can be kept for 7 days in the refrigerator or 24 hours at room temperature. After these timeframes, it is not guaranteed to be free of microbes. It is important to use sterile water, without any microbes, but it doesn't necessarily have to be free of minerals like distilled water. Bottled water in smaller sizes could work as well. Alternatively, you could boil water for at least 5 minutes, let it cool, and it will also keep for 7 days in the fridge.