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SB 10.56.24

āsīt tad aṣṭā-vimśāham
 aviśramam ahar-niśam
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.
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.

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