「いつまでに？」の表し方

「いつまでに？」という締め切りのたずね方，答え方をいくつかのパターンでネイティブに確認してもらったときのものです。

紫字がネイティブ（オーストラリア人）によるものです。

1. When do you need it by? [Correct.]

2. A: When do I have to finish this? [Correct, although Americanised. “When do I have to finish this by?” is traditional English.]

　 B: By this weekend. [Correct.]

3. A: When do I have to finish this by? [Correct.]

 　B: This weekend. / Tomorrow morning. / Next Monday. / By next Monday. / In one hour.

[Responding to the question with “By next Monday” is the best English, since you’re really saying, “You have to finish it by next Monday.”

Omitting the ‘by’ is not wrong, but sounds a tad clumsy to my ears.

“In one hour” is correct, because the listener does not need to be alerted to the change in logic. However, it would be better to say, “Give it to me in one hour.”]

Americanaisedとありあますが，
How long are you going to stay?に対するFor two weeks. / Two weeks.
また，
The festival lasts three days. vs. The festival lasts for three days.
のような，前置詞forの省略もA.E.の特徴のようです。

なお，By when do you need it? のような，〈前置詞＋疑問詞～?〉の形も文法書などで見かけますが，実際の日常会話では堅すぎると思います。