for many years; for years
Emphasizes a long duration, often with a sense of 'for years on end' or 'for so many years'. Used adverbially.
He worked abroad for many years.
I reunited with a friend I hadn't seen for years.
Combination of 何年 (how many years) + 間 (interval/duration) + も (inclusive particle), literally 'for even how many years', expressing a long, indefinite period.