This is the 2057th most frequent Korean word.
수년간
"For many years."
In this sentence, "수년간" is used to indicate a period of several years during which the subject has been engaging in the activity mentioned.
그는 수년간 이 프로젝트를 준비했다.
He has been preparing this project for years.
Here, "수년간의" acts as a descriptive phrase modifying "연구", specifying the lengthy duration of the research.
수년간의 연구 끝에, 우리는 중요한 발견을 했다.
After years of research, we made a significant finding.
This shows "수년간" being applied to highlight the time span during which the subject undertook the described actions.