for a long time; for quite some time; long interval
Used as an adverbial phrase or noun to indicate a long duration. Often appears with particles like に or の, or as a standalone adverbial.
Thank you for everything over the long time you've helped me.
彼は長い間、海外で暮らしていた。
He lived abroad for a long time.
長い間の努力が実を結んだ。
His long-time efforts bore fruit.