For many years; over a long period of years (used to indicate continued duration).
彼は何年間も海外で仕事をしていた。
He worked abroad for many years.
この家は何年間も空き屋だった。
This house had been vacant for many years.
何年間も練習を続けて、ようやくプロになれた。
After practicing for many years, I was finally able to become a professional.
その問題は何年間も解決されずに残っている。
That problem has remained unresolved for many years.