SB 10.56.24

āsīt tad aṣṭā-vimśāham
 itaretara-muṣṭibhiḥ
vajra-niṣpeṣa-paruṣair
 aviśramam ahar-niśam
Word for word: 
āsīt — was; tat — that; aṣṭā-viṁśa — twenty-eight; aham — days; itara-itara — with one another’s; muṣṭibhiḥ — fists; vajra — of lightning; niṣpeṣa — like the blows; paruṣaiḥ — hard; aviśramam — without pause; ahaḥ-niśam — day and night.
Translation: 
The fight went on without rest for twenty-eight days, the two opponents striking each other with their fists, which fell like the cracking blows of lightning.
Purport: 

Śrīla Viśvanātha Cakravartī points out that the fight continued day and night without intermission.