How long boiled water “lasts” depends on how you store it and what you mean by “last”:

1. At normal room temperature (in a clean, covered container)

• Safest to use within 24 hours.
• After that, risk of re‑contamination from air, utensils, or container increases.

2. In the refrigerator (in a clean, covered container)

• Generally fine for 2–3 days.
• Always use a clean vessel and don’t dip used cups/spoons into it.

3. In a thermos/flask

• It usually stays hot for only 6–12 hours, depending on the flask.
• Microbiologically, treat it like room‑temperature stored water: best within 24 hours if the flask is clean and kept closed.

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Important hygiene tips

• Use a clean, preferably boiled first container (glass or steel is better than plastic for hot water).
• Keep it covered and avoid frequent opening.
• Don’t touch the inside or mouth of the container.
• If it smells odd, looks cloudy, or you doubt cleanliness, don’t drink it—boil fresh water again.

So, as a simple rule:

• Best practice: finish boiled water within 1 day at room temperature, or within 2–3 days if refrigerated properly.