several years; for several years
Used as a noun or adverb to indicate a period of a few years. Often appears with particles like に or の, or as a standalone adverbial.
彼は数年間海外で働いていた。
He worked abroad for several years.
数年間の努力が実を結んだ。
Several years of effort bore fruit.
Compound of 数 (several) + 年 (year) + 間 (period). A straightforward Sino-Japanese temporal expression.