"When will you study?"
the right word order can really be a problem, at least for the beginner ... In this case, Duolingo demands position of いつ at the beginning and rejects 勉強いつしますか. Just one or two sentences before, it refused to accept the analogous word order いつマリアさんはにほへ来ますか ("when will Maria come to Japan?") and insisted on マリアさんはいつ etc. So what?
that's because in the「『マリアさんは』いつ日本に来ましたか？」example, the "question word" いつ is placed at the start of the sentence, but right after the topic. Usually topic first sounds more natural because before saying something about something, you should declare what that something is, logically speaking. That said, I found that in practice, natives do sometimes phrase it as you did, so it's not that unconventional.
You can find similar examples here: