Number + 天
天 (tiān) means "day" and is used as a measure word to count days. It appears directly after a number without needing a separate classifier, as in 三天 (three days). It is one of the most common time words in daily Chinese.
我在那座城市待了五天,玩得很开心。
wǒ zài nà zuò chéngshì dāi le wǔ tiān, wán de hěn kāixīn.
I stayed in that city for five days and had a great time.
这个项目还需要三天才能完成。
zhège xiàngmù hái xūyào sān tiān cáinéng wánchéng.
This project still needs three more days to be completed.
她每天只睡六个小时,太辛苦了。
tā měitiān zhǐ shuì liù ge xiǎoshí, tài xīnkǔ le.
She only sleeps six hours a day; it's really tough.