As Tung already said, they all can be used interchangeably. "Lúc nào" is another word that means exactly the same thing. However, I want to point out some tricky things. If you want to ask when something happened in the past, use "khi nào" or "lúc nào" or "hồi nào." When did you see him? Bạn thấy anh ta khi nào (lúc nào) (hồi nào)? Using "bao giờ" in this case would make it sound a little awkward but understandable. Using "chừng nào" would be wrong. Also, note that"hồi nào" can ONLY be used for past events. When is he coming? Hồi nào anh ấy (sẽ) tới? -> WRONG (use the other 4). So, you may ask, "But how do I know when they mean past-when and when they mean future-when because Vietnamese doesn't conjugate verbs?" The answer is 1) context and 2) when they appear at the end, they mean past-when and vice versa. For eg., When did you see him? Bạn gặp anh ấy khi nào? When will you see him? Khi nào bạn gặp anh ấy? When you see him, tell him I say hi. Khi nào bạn gặp anh ấy, cho tôi gởi lời hỏi thăm. Yeah I know. Vietnamese is tricky like that. =)

And the answer is you can use whatever you want :) It doesn't matter at all. The sentence "When will he come?", for example, can be translated as followings: 1) Chừng nào (thì) anh ấy (sẽ) tới. 2) Bao giờ (thì) anh ấy (sẽ) tới. 3) Khi nào (thì) anh ấy (sẽ) tới. However, people in different areas would use different words which mean "when". In the north of Vietnam, we tend to use "bao giờ" and "khi nào" Hope it helps! :)

Those 3 are "when", exactly the same meaning, but: 1. used in southern accent 2. used in northern accent 3. standard Vietnamese

Thanks!

They are all interchangeable. "Hồi nào" is only usable if the event happened in the past. "Hồi" is a Han-Viet word (Chinese origin) 回 means "return". If it is future, don't use "hồi nào" since it's only for past event. Besides, all future events use the word "sẽ" (will). Hồi nào is only for past event. Simple as that.