A few years ago I tried breathing very slowly. I don’t remember why, maybe after reading some article on meditation. Eventually I tried breathing as slowly as possible (i.e. as slowly as could be sustained indefinitely without developing air hunger), and found that I could sustain one breath per minute for as long as I wanted.
The best technique I found was to inhale steadily for thirty seconds, then exhale steadily for thirty seconds (as opposed to inhaling relatively quickly and then holding your breath for ~1 minute). I needed to watch a clock to make this work.
So…how slowly can you breathe?