• A single Upvas (fast) in Shwetambar Jain practice is generally one day, i.e., about 24 hours.
• Start: after the last meal of the previous day (usually from dawn/sunrise).
• End: with the first meal after the next dawn/sunrise the following day.
• Note: some households or regional communities observe it from sunrise to sunrise, while others follow a sunset-to-sunset convention. Both are common depending on family tradition.

If you’d like, I can point you to a specific Shwetambar Upvas reference from Jain sources for more precise ritual details.