This is the 438th most frequent Dutch word.
jarenlang
For years.
Here, 'jarenlang' is used to emphasize that the action (working as a teacher) occurred continuously over an extended period.
Hij heeft jarenlang als leraar gewerkt.
He worked for years as a teacher.
Here, 'jarenlange' is an adjective describing the longevity of the friendship, indicating it lasted many years.
Een jarenlange vriendschap werd uiteengerukt.
A friendship that lasted for years was torn apart.
In this sentence, 'jarenlang' highlights the duration of time that passed in the context of completing the project.