In Jainism, the duration of suffering in hell (Naraka) is determined by one's accumulated karma, particularly papa (bad karma). Fasting (upavasa) and other austerities (tapas) are highly regarded as means to destroy accumulated karmas and prevent new karmas from binding to the soul.

However, Jain scriptures do not specify an exact number of years by which one's stay in hell is reduced through fasting. The effect of fasting depends on the intensity of the austerity, the purity of intention, and the overall spiritual progress of the individual. Fasting is seen as a way to purify the soul and lessen the effects of bad karma, potentially reducing future suffering in hell or even preventing rebirth there.

To summarize: Jainism teaches that fasting and penance can lessen bad karma and the likelihood or duration of hellish existence, but it does not quantify this in terms of specific years. The reduction is qualitative and depends on the individual's sincere efforts and inner transformation.