The first of the eight major hells is called the Hell of Regeneration, which is located 1,000 yojana (9,600-24,000 km) beneath this world (Jambudvīpa) and it occupies a vast area of 10,000 square yojana. Sinners who fall into it hate one another and fight like dogs against monkeys whenever they happen to meet. With iron fingernails, they scuffle with one another causing bleeding and injury to the flesh, until nothing but skeletons remain. Or they are beaten up by hell guards with iron bars until their whole body from head to toe is crushed into particles of sand or cut up by a sharp sword into small pieces. Upon dying from such unbearable sufferings as these, they will be regenerated to undergo these cruel torments again.

The life span of these sinners in this hell is as follows. Suppose 50 years in the human world is equal to one day in the Four-king Heavens, the first heaven in the realm of desire, where heavenly beings’ life span is 500 years. Suppose 500 years in the Four-king Heavens correspond to one day in the Hell of Regeneration. Sinners in the Hell of Regeneration live as long as 500 years.

Ken Hōbō-shō, A Clarificaton of Slandering the True Dharma, Writings of Nichiren Shōnin, Doctrine 3, Pages 106.