Simple, traditional rule (for laypeople doing upvaas with water):

• Start: After Navkarsi, i.e., 48 minutes after sunrise. Do not drink at or immediately after sunrise.
• Stop: Before sunset. Best practice is to finish at least one antarmuhurt (about 48 minutes) before sunset. No water at night.

Notes by tradition:

• Shwetambar: During sajala upvaas, only boiled, filtered water may be taken between Navkarsi and sunset.
• Digambar: Ideal upvaas is nirjal (no water). If water is permitted by your maryada, keep it to daylight hours; some groups keep even stricter limits (e.g., stopping earlier).

Practical maryada:

• Take pachkhan in the morning stating how many times you will drink.
• Use only boiled and properly filtered water; avoid using it beyond a day. In stricter observance (e.g., Paryushan), many prefer freshly boiled within a shorter window.
• Health exceptions are allowed under guidance.

References: