In Argentina at least, it can be understood as "What are you doing?".

Andoni is absolutely right. In Spain it doesn't means anything, furthermore, it's deeply incorrect, but in some other Spanish speaking countries it's a way to say: "You do a lot of things, don't you? It's all that necessary ? Example: You ask a friend to check the oil level of your car, and he starts to remove pieces from the engine. Then you say: "¿¡Que tanto haces!? ¿Solo te pedí que midieras el aciete?

In addition to my previous answer, since the question "¿qué tanto haces?" has a the object the second singular person, I have to add that if the question is directed not to an object, but to a person, it could also mean "how much can it be requiered from you?" or "how much weight can you lift?". In other context, refering to the activities of a person in his or her personal life "no sé qué tanto hace Cristóbal en la casa de Sabrina" (I don't know what Cristóbal does at Sabrina's place")

Even though, I do agree with the explanations above, there is one case I can think of where it is completely right (even grammatically) to use it. if you are talking about the maximum space of a bucket or bottle, the question "¿qué tanto hace?" will refer to the maximum amount of liquid that it can be filled with. Eg. "tengo una botella, ¿te sirve?" "¿depende de qué tanto haga?" ("I have a bottle, is it good for you?" "depends on how much I can fit in it"). Cheers, Mario

this phrase is not spanish. and if you use as stated before, probably you have to do some other question about. i suppose it is a kind of usa spanish meaning How do you do?, probably slang or from disglossic people. (it is curious to see the explanations above which clearly shows how the phrase is unintelligible as it is).