The longest my hair has ever been was down to my waist.
I let it grow all the way down to my waist once.
この場合、次のような言い方ができますよ。
ーThe longest my hair has ever been was down to my waist.
「髪を一番長くしたのは腰までだ」
ーI let it grow all the way down to my waist once.
「一度腰まで髪を伸ばした」
例：
A: What's the longest your hair has ever been?
「どのくらい髪を長くしたことがある？」
B: The longest my hair has ever been was down to my waist.
「一番長くしたので腰まで」
ご参考まで！
カナダ 
