I'm personally following the PPH guidelines which recommend, in general a flow rate of 15/L per minute—depending on your personal rate of breathing (taking into account your pulmonary health)—if assumed to be "normal" a 20cubicfoot tank should last 30 minutes or so.
I'm a smoker, a pretty heavy one these days, but I self-monitored my breaths per minute and fell into the normal range of 12-20 per min.
Heart conditions, pulmonary conditions, etc influence the breathing rate.
What size is your tank? I don't understand what you cited...