1. Until when will you stay at the office?
2. Until when do you stay at the office?
I think both are natural but I read in time clauses we are not allowed to use will after when.
Personally, a lot more natural to me is:
"How late" not "Until when"
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.