Short answer: 8 prahars.

Details:

• 1 prahar ≈ 3 hours.
• Shwetambar common maryada: boiled water (ukalela pānī) is considered usable for up to 8 prahars (one full day–night) from the time it was boiled; however, it is actually consumed only during daylight (sunrise to sunset). After the limit, re‑boil fresh water.
• Stricter observance (especially in Chaturmās): some sanghs limit it further to a few prahars; once the fixed prahar limit ends, do not use without re‑boiling.
• Digambar practice: generally only freshly boiled/filtered water during daytime and not kept overnight; follow your guru‑paramparā’s maryadā.

For simple explanations from JainKnowledge:

• Ukalela pānī no kāl
• How long can I use boiled water