What is the longest you've gone without washing your hair?
"今まで最長何日間髪の毛を洗わず過ごした？"
- What is the longest you've gone without washing your hair?
- What is the longest period of time you have gone without washing your hair?
- What's the longest you've ever gone without washing your hair?
What's the longest you have gone without washing your hair?
What's the maximum number of the days you have gone without washing your hair?
この場合、次のような言い方ができます。
ーWhat's the longest you have gone without washing your hair?
「今まで最長何日間髪の毛を洗わなかったことがある？」
have gone without washing your hair で「髪を洗わないで過ごす」
ーWhat's the maximum number of the days you have gone without washing your hair?
「今まで最長何日間髪の毛を洗わなかったことがある？」
What's the maximum number of the days ....? で「最長何日間…？」という聞き方です。
ご参考まで！
アメリカ合衆国 
カナダ 
