Thanks for the explanation. For a date we might say "What date are you due back?"
It would be just as natural to respond to "How long are you going to be away?" with "until September 30th" as it would be to say " for three weeks". It does not require a duration as the response, It "invites" either a date or a duration, as both essentially convey the exact same information.
IMHO this would be a better translation than "What date are you returning" which is pretty far from the original German, or "When are you away until?" which is just awful English.
The English sounds very wrong to me. "When are you gone until?" sounds better. It's probably grammatically incorrect, but it's something people would actually say, at least in my area (Midwestern Untied States, where we use prepositions to end our sentences with). I can't picture a native speaker ever saying "Until when are you gone?"
You could, in English, virtually keep the same German word order and almost keep the direct translation with a slight tweak to the use of 'bis'.
Von... bis is the from ... 'to' usage rather than from... 'until'. So you could say a enquiring quizzical question 'To when are you away?'