In his book Breath: The New Science of a Lost Art, James Nestor describes  the optimal breathing cycle as the 5.5s breath practice. This pattern of “5.5-second inhales followed by 5.5-second exhales, which works out almost exactly to 5.5 breaths a minute,” synchronizes with the rhythms of blood pressure, heart rate and respiration patterns of 5.5-second oscillations. This cardio-respiratory synchronization has beneficial impacts on your heart rate variability. The heart rate variability (HRV) came to our collective awareness when poly-vagal theory became popular. HRV is the fluctuation of time between heartbeats. A high HRV is optimal for physical and mental health; a low HRV may indicate potential concerns. 

It is highly suggested that you breathe through your nose as often as possible. We are meant to breathe through our nose as it is biologically designed to warm and add moisture to our air for optimal oxygen absorption. There are so many other benefits to nose breathing; I recommend Nestor’s book mentioned above if you are interested.