"When will you come from Delhi?"
Translation:तुम दिल्ली से कब आओगे ?
The simple answer is, Hindi question words, like most other words are fairly flexible where they can go in a sentence. But for question words like क्या, कब, कौन, etc., the two best options are either (i) right before the verb or (ii) the same place you would want to put the non-question version. You can see that the default version given here is (i), but since आज would go happily right after the subject in this sentence, कब can also go there - and hopefully Duo accepts that one!