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.
幾日も雨が降り続いた。
It rained for many days on end.
彼は幾日も家に帰らなかった。
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'.