This is really dependent on the result you are looking for. The following describes which breath rates are optimum for each of the areas:

• Heart Coherence: Studies show that heart coherence is increased most by breathing at a rate of 5 or 6 breaths per minute (5 seconds in, 5 out) with an equal I:E (inhale:exhale) ratio. However coherence still occurs with any ratio as long as you breathe rhythmically. One of the goals of pranayama is to bring your normal day to day breath rate down to at least 5 or 6 breaths per minute, or even half that. According to yoga – longevity is dependent on your breath rate – the less you have to breathe, means the more efficient your physiological functions are, and so less the impact of oxidative stress, and the longer you live
• Flow States: Heart coherence and getting into flow using your breath depends on rhythm, smoothness and intention. An active breath rate of 6-10 breaths per minute is adequate for creating the higher energetic states needed for positive emotions like passion and joy. Passion is the number one predictor of flow and peak performance. More rapid breathing should always be followed by breath retention (kumbhaka) in order to bring the physiology back into balance
• Rest & Digest: Studies show that by extending the exhale time e.g breathing in an I:E ratio of 0.5 results in a more of a relaxation response compared to just slowing down the breathing rate. Your parasympathetic nervous system switches on which controls your rest and digest mode. Do a few rounds of this before and after eating to avoid indigestion.

The Ideal I:E Ratio (Inhale:Exhale) = 0.5

According to the Yoga sutras, the ideal way to breathe is with a ‘slow rechaka’. Rechaka means expiration in sanskrit. The ideal ratio is your exhale being double your inhale time = 0.5. This ratio is also shown in studies to be the optimum time needed for increased relaxation, stress reduction, mindfulness and positive energy. This can be explained physiologically as exhalation stimulates the parasympathetic nervous system. You also absorb most oxygen on your exhale, so this also adds to the increase in positive energy.

When you breathe in this ratio, the conscious focus and effort required to extend the exhale stimulates the cerebral cortex. This is a very important region of the brain that deals with things like thought, awareness, memory, attention and language. By bringing your attention to slowing down your exhale, your thoughts become more focused, your parasympathetic nervous system is switched on and your emotions become more calm.