In Jainism, the duration of a fast can vary a lot:

• Short fasts: a few hours or a half-day.
• One-day fasts (upvas): usually from sunrise to sunset (roughly one day).
• Longer or more intense fasts: several days or more, often as part of special vows guided by a guru or spiritual instructor.

So, there isn’t a single fixed length—fasts can be brief or extend across multiple days depending on the vow and practice one follows.