For a long time; an extended period. Used adverbially or nominally to indicate a long duration.
長い間待った後、やっと電車が来た。
After waiting a long time, the train finally came.
彼とは長い間友達だったが、最近連絡が途絶えた。
We were friends for a long time, but recently we've lost contact.
そんなことを長い間知らなかった。
I didn't know that for a long time.