I would say "What time is your appointment?" if it's for an appointment. If it's for something more casual and you know the location (example-meeting) "What time is the meeting?" If you do not know the location/event "What time are you supposed to be there?" should be sufficient.

What time is your appointment? I mean this is by the far the easiest way. What time do you have to be there? Edit: I'll add: When is your appointment? When do you have to be there? Those work fine too. Being honest, I wouldn't use any of your questions. The thing you want to know is the time they will get there, and your questions are rather awkward to my ear, bearing that in mind, as they make no reference to that piece of information. 'When are you to be there?' sound really formal. No one would say it. 'When are you supposed to be there?' is OK, it's the best of the three but I wouldn't use it in preference to the questions I wrote above. 'When are you going to be there?' Again, it just sounds off. I'll tell you the context in which I would use it. Say for example, someone I have just met has told me they are going on a trip. I might then say, 'Great, so when are you going to be there?' It's making reference to a longer, more open-ended time, not a more immediate, fixed time, like in your example.