"We arrive approximately an hour later."
Translation:Arriviamo approssimativamente un'ora dopo.
No, it would be uncommon but not a grammar mistake, so don't expect any scandalized reaction; in fact, a long time ago it was recommended to split "una ora" when hyphening. Nowadays it's more common to hyphen as "un'o-ra" or not hyphening at all, perhaps because usually word processors aren't able to properly hyphen in presence of apostrophes. Other than that, not truncating is often a stylistic choice when marking words, often during a public speech.