Short answer: Use it within the same day, and never beyond 24 hours.

Details

• Shvetāmbara tradition (general lay practice during Paryushan): Boil and filter in the morning; use that water for ekasana the same day, ideally finishing it before sunset. The upper limit allowed is 24 hours (from that morning to the next morning). After 24 hours, discard or re‑boil.
• Digambara tradition (general lay practice): The same practical maryādā is followed—freshly boiled and filtered water is used the same day; do not keep it beyond 24 hours.

Stricter vows: Many observe a tighter limit (12 hours or even 6 hours). During Paryushan, taking the stricter option is encouraged if feasible.

Essence: Freshly boiled, properly filtered, covered, and consumed the same day—preferably before sunset; absolutely not beyond 24 hours. Always follow the maryādā of your guru/derasar if it is stricter.