The perfect breath is this: Breath in for about 5.5 seconds, then exhale for 5.5 seconds. That's 5.5 breaths a minute for a total of about 5.5 liters of air. You can practice this perfect breathing for a few minutes, or a few hours. There is no such thing as having too much peak effciency in your body.
Excerpt from the book "Breath, the science of a lost art" by James Nestor - Very recommended