About a week, if this really is just another standard experiment.
[Frankly, now that he stops to think about it, he isn't too worried about what Mithos will do to him while he's like this. Yuan is inside Mithos' own body, which basically ensures his safety. However, there's absolutely nothing protecting Yuan's own body. Mithos wouldn't be stupid enough to run a sword through his gut, of course, but there's plenty of things to do to him that don't involve killing or maiming.]
[Action]
[Frankly, now that he stops to think about it, he isn't too worried about what Mithos will do to him while he's like this. Yuan is inside Mithos' own body, which basically ensures his safety. However, there's absolutely nothing protecting Yuan's own body. Mithos wouldn't be stupid enough to run a sword through his gut, of course, but there's plenty of things to do to him that don't involve killing or maiming.]