several decades; many decades; a few decades
Refers to a period of roughly 20 to 90 years, often used when the exact number is unknown or not specified. Can be used as a noun or adverbially.
This town has changed greatly over several decades.
I returned to my hometown for the first time in decades.
Compound of 数 (すう, 'several') + 十年 (じゅうねん, 'ten years').