In Jainism, fasting and austerities such as upvas (fast days), chivihar, ekasana (one-plate/one-meal regimen), and other tapas are said to reduce the karmic bondage you accumulate. They help burn (or shed) karmas and thereby lessen the suffering one experiences in the next life, including the duration of stay in hell. However, there is no universal, fixed number of years given in the Jain scriptures. The amount of karma shed depends on factors like the intensity, sincerity, and duration of the austerity, as well as one’s mental attitude and previous karmic balance. The key idea is that greater self-discipline and pure intention yield greater karmic reduction, leading to shorter or avoided suffering in future lives. There are differences in emphasis between Digambar and Shwetambar traditions, but both affirm that sincere fasting and ascetic practice help reduce karmic bondage.

If you’d like, I can look up specific sutras or passages from Jain scriptures that explain how tapas reduces karma and how it affects future lives. You can read related explanations on JainKnowledge.com.