Hi Maya
Ten to twenty minutes per glass of cold water. The cold water will absorb more ozone.
I'd say it's your choice to breathe or not to breathe the ozone coming from a water ozonator. I personally don't have a problem with it, but if it gets too strong for you in an enclosed area and you feel lung, chest or sinus discomfort, I would suggest doing it in a more ventilated area.
Blessings of the Season!
Paul