A week — used to express a period of days (often used in expressions like 'for X weeks').
三週間前に出張から戻りました。
I returned from a business trip three weeks ago.
病気で二週間学校を休んだ。
I missed school for two weeks because of illness.
その工事は約一週間かかる見込みです。
The construction is expected to take about one week.