for many days; for days on end
Emphasizes that something continues or lasts over a period of many days, often with a sense of duration or repetition. Commonly used with negative expressions or verbs indicating continuation.
It rained for many days on end.
He didn't come home for days.
day after day; for days on end
Combination of 幾 (iku, 'how many') + 日 (nichi, 'day') + も (mo, 'also/even'), literally 'even how many days', used adverbially to mean 'for many days'.