I've read that one should use "ir a" when time of completion is certain. If the time is uncertain, one should use the future tense. This choice is not so clear-cut when asking a question. Take for example, asking "When are you coming?". You're expecting an exact time from the other party. But the other party may not know the exact time or whether he can come at all.
¿Cuándo vas a venir?
So which one do you use when one side wants a precise time while the other side cannot be guaranteed to provide an exact time?