"It takes one day."
A theory says when time indicator is used like an adverb, it does not require a particle.
It takes a lot of time.
It takes 3 hours' time.
I guess Japanese uses it so naturally that there is very little discussion about it. it seems to be only confusing for foreigners. Anyway learn the pattern by memorizing it first and don't be too astonished when you see cases with a particle after time, which can be は, で, が, に and others.
In every case the kanji is read the way duo wants it typed. That's what makes people frustrated. Then in some places, they ONLY accept the kanji.
The worst is days of the week. 月曜日 is wrong and so is げつようび. Instead, it needs to be typed it in three parts, makings sure that the middle part is in hiragana instead of kanji while the first and third parts are in kanji, not hiragana.
The Japanese learning English course is much more lenient.
EDIT: They've started accepting 曜日, so that's nice.