An adverb meaning 'for many days' or 'for several days', used to indicate something continuing over multiple days.
彼は風邪で何日も寝込んでいた。
He was bedridden with a cold for several days.
何日も考えた末、進学を決めた。
After thinking for many days, I decided to go on to higher education.
荷物が届かず、何日も待たされた。
The package didn't arrive, and I was made to wait for several days.