"When do they arrive?"
Translation:Elles arrivent quand ?
What you propose is not an interrogative format per se. It is constructed as a statement with just a question mark at the end.
Actually "quand ils arrivent" is the form of an indirect question, like: "I want to know when they arrive / Je veux savoir quand ils arrivent", instead of "I want to know: 'When do they arrive?'" / "Je veux savoir: 'Quand arrivent-ils ?'".
"quand est-ce que" is not recommended by the Académie Française, because you add 2 interrogative words to start your question. But in speech, you will hear it often (+ "où est-ce que", "pourquoi est-ce que", "quand est-ce que", etc.)
In real life, yes, if context allows (maybe the question is about a day or a week, not specifically about "à quelle heure").
Why does it have to be elles and not ils? Also why can't I put quand at the start of the sentence?
Duo accepts "ils" or "elles" since "they" has no gender.
You may ask the same question starting with quand:
- quand arrivent-ils/elles ? (formal interrogative format)
(dis-moi) quand ils arrivent = (tell me) when they arrive.
so it is not a question and does not correctly translate "when do they arrive?"
"quand" ends with a consonant and naturally liaises (with sound T) with the next word starting with a vowel sound, without a need for a dummy T nor a hyphen.
But "arrivent-ils" is an inversion verb-subject that needs a hyphen.
La voix feminine prononce le mot "arrive" comme une americaine, pas comme une française. Pas bien du tout, si on veut s'apprendre la langue française.