Particles after time expressions like "三週間"

Question

I am a beginner in Japanese so I was trying to write some simple sentences to practice my vocabulary and grammar.

I wanted to write the sentence:

My friend is in Japan for three weeks.

as:

私の友達が　三週間　日本に　います。

I am unsure if I should put any particle after 三週間 and whether this sentence is grammatically correct at all.

Answer 1

[間]{かん} itself is functioning as a suffix indicating 'interval' or 'period of time'. In other words, '三週間' literally translates to '三週 (three weeks) + 間 (during or for)'. Therefore, you don't need any other particle or suffix after '三週間'.

One more important thing you should note is Japanese normally doesn’t put spaces between words.

Answer 2

You could insert:

   私の友達が三週間のあいだ日本にいます。

   私の友達は、三週間の間 日本にいます。

三週間の間 sounds redundant but it's correct.

Many people insert に making it "...週間の間に" but it's often incorrect (sloppy diction).

This first example (below) is ungrammatical / illogical -- or especially sloppy.

【衝撃】たった4週間の間に毎日3Lの水を飲み続けた女性の変化がヤバすぎ！ 皆さん、普段の生活の中で意識して”水”を飲 ... .......

      ( I suppose she got prettier and/or younger-looking. )

緊急警告！！一週間の間に「東京大地震」がくる可能性があります。