Quand reviens-tu? (formal)
Quand est-ce que tu reviens? (informal)
Here's a breakdown:
* Quand: When
* reviens: (formal singular "you") returns
* est-ce que: (informal question word) is it that
* tu: (informal singular "you")
* reviens: (informal singular "you") returns
You can also use "Quand rentres-tu?" (informal) which literally means "When are you coming home?" but can be used as a general "When are you back?" in many contexts.
The choice between formal and informal depends on your relationship with the person you are speaking to.