"When would you turn back on a journey" is questionable, at best, because it really doesn't make much sense. Notwithstanding this, it likely doesn't express what Mimi would like it to. As Winklepicker stated, to "turn back" usually means to set out for a destination, get part of the way there, change your mind and return to your point of departure. Example:Both of the sentences are correct and can not be interchanged.
It's great for your example.We will go on a bike ride, starting at noon.
--What time do you want to get back?
Well, I'd like to get back by 3:00 so that I can start dinner.
--Okay, then however far we've ridden, we'd better turn back around 1:30 in order to get back by 3:00.