If you listen to any tango composition, you will feel a rhythm of a different kind – its breath rhythm – meaning, the way you would want to breathe, while dancing to this piece. For example, your breath rhythm may match the phrases of the composition, where every inhale-exhale corresponds to one musical phrase, but this is not a rule. It makes sense that things like this are subjective, and a composition will not have a "correct" breath rhythm. Your rhythm of breathing will be individual, and will depend on many factors during the dance – it's only important to consciously "listen" to the music with your breath. Try this – pick a composition, and breathe to it without dancing.
When to inhale and when to exhale? Tango music often exhibits a very clear dynamic of swells – increasing and decreasing levels of energy. Many elements play into this: volume, tonality, change of tempo, emotional component, dominating instrument, etc. However, these increases and decreases of energy are easier felt than understood. Personally, I like to inhale when the overall energy increases, and to exhale when it decreases.