"It takes one or two days."
Translation:Det tar en eller två dagar.
It's not technically wrong, but we don't often use it like that. If something takes more than one day, it's sort of implied that it'll take the night as well, so unless we have a specific reason to point out that it's the full 24-hour dygn, we mostly just use dagar.
Bah! Okay, I though that when specifically using it as the number one rather than as 'a' it was supposed to be ett.
Think of it like a "silent" dag in between, like this: Det tar en (dag) eller två dagar.
But instead of repeating "dag" twice, you only have to say it once (same in English, it would be cumbersome to say "It takes one day or two days").