Short answer: Jain scriptures do not give any fixed “number of days of Narak (hell)” reduced by one Upvas.

Authentic teaching:

• Upvas (fasting) is a tapas that does nirjarā (shedding) of karmas. By reducing passions and practicing samyata, it can lessen the intensity and duration of future suffering, including Narak, but the result depends on one’s inner intention, discipline, and purity—not a fixed count.
• Both Shwetambar and Digambar traditions agree that tapas destroys karma; neither tradition’s canonical texts prescribe a calculation like “1 Upvas = X days less in Narak.”

If you have heard exact numbers, those are popular sayings, not from Agamas or Tattvartha-style siddhānta.

For a simple overview on fasting/tapas and why it helps in nirjarā: Also: “How many years of hell reduce by doing fast?” explained here: