In Śvetāmbara Jainism, the 4th āra of Avasarpinī (the descending half) is called Duḥṣamā-susamā. Its duration is the long, cosmic measure: 1 koṭi-koṭi sāgaropama years, minus 42,000 human years. In other words, it is essentially “one koṭi-koṭi sāgaropama years” with a subtraction of 42,000 years. This is the traditional Śvetāmbara view of the fourth āra. The general framework of six aras in both Śvetāmbara and Digambara traditions is the same, though some historical dates may be stated slightly differently in different texts. You can read a concise summary of the six aras here and a related overview here .

You can read more about how the six aras are described in Jain cosmology here .