also: いくとせ
noun, adverb
number of years; how many years
Used to ask or talk about an unspecified number of years, often in questions or expressions of duration. The reading いくとせ is more literary or poetic.
この町に住んでから幾年になりますか。
How many years has it been since you started living in this town?
幾年もの間、彼はその研究を続けてきた。
He has continued that research for many years.
何年 is the more common and direct way to ask 'how many years', while 幾年 is more formal or literary.
Compound of 幾 (iku, 'how many') + 年 (nen, 'year'). The reading いくとせ uses the older native Japanese counter とせ for years.