many years; for many years
Often used in formal or written contexts to indicate a long period of time. As an adverb, it can modify verbs to mean 'for many years'.
Drawing on many years of experience, I tackled the new project.
He has contributed to the development of this region for many years.
多年 is a bookish, literary synonym for 長年, both meaning 'many years', but 多年 is rarely used in casual speech.
多年 is a more literal, formal word for 'many years', while 万年 emphasizes an extremely long or seemingly endless duration, often rhetorically.
Compound of 多 (many) and 年 (years). The reading たねん is the on'yomi for both characters.