Translation:The girl comes back from school at two.
Right. For 2,3,4, you use singular genitive часа and for 5-20 the plural genitive часов - when the number is in nominative or accusative inanimate case as here. It gets more complicated when the number is in another case, see: https://www.youtube.com/watch?v=rUYzNgGy_HU&list=PLrIkLgUgjNHcq6hhO_btxkoFghnXeMZul&index=6
I believe duo grades responses based on whether they fit into the possible options programmed in. So generally you probably don't want to add extra words that aren't in the original translation since you can't assume the people inputting the answers would think of every random additional word people might decide to add.
The English sentence is usually a good indicator of what was in the original.
That said, we usually accept both "Мне двадцать лет" and "Мне двадцать", "Сейчас два часа" and "Сейчас два" regardless. Same for English ("I am 20 years old" or simply "I am 20").
Note that 1 o'clock is "один час" OR "час"—not "один".