for many days; for days on end
幾日も雨が続き、川の水位が上がった。
It rained for days on end and the river level rose.
彼は風邪で幾日も寝込んでいた。
He was bedridden for several days with a cold.
幾日も考えた末に返事をした。
After thinking for many days, I gave my answer.