noun
hundred days; lengthy time
Literally 100 days; often used figuratively to mean a long period of time, especially in set phrases like 百日の説法屁一つ (a single fart ruins a hundred days of preaching).
百日間、毎日練習を続けた。
I continued practicing every day for a hundred days.
百日の苦労が水の泡になった。
A hundred days of hard work came to nothing.