Chauvihar Upvas is the stricter form of upvaas (no food and no water after sunset until the next sunrise). Some traditional notes (not uniformly fixed in canonical texts) say that performing Chauvihar Upvas can shed a very large amount of karma, sometimes described as 1000 billion years of hell. However, authentic Jain scriptures do not give a fixed numeric reduction for Narak, and the exact figure can vary by tradition (Digambar vs. Shwetambar) and by acharya’s guidance. The key point is that upvaas helps burn karma and reduce future suffering when done with the right faith and discipline, not by a precise year-count. You can read more about Chauvihar Upvas and its distinctions here and about the difference between Tivihar and Chauvihar Upvaas here