long period of time; many years
Refers to a span of many years; often used in contexts like 'for many years' or 'over a long period'.
He has worked at this company for many years.
Years of effort have borne fruit.
長期間 is a general term for 'long period'; 永年間 carries a nuance of permanence or an extremely long duration.
Formed from 永年, “many years” or “a long time,” and the duration marker 間.