“When will you be leaving” vs "When will you leave"
When will you be leaving? (future continuous) Polite question-you are asking in a nice way-it may mean you would like to know when the person is planning to leave.
When will you leave? (future simple) Request-basically you are indicating you want that person to leave.
If someone is moving to a new job, or going on holiday, or making a business trip, and the answer will be "Sometime next week," "In a month or two," then you might ask,
"When do you leave?"
"When will you leave?
If you expect a precise answer, "At six o'clock." "At seven to-morrow morning," then you would be more likely to say,
"When will you be leaving?"