I tried breathing as slowly and deeply as I comfortably could, and got 2 minutes 20 seconds for four full breaths without much effort. And I'm not even very athletic. Maybe I should try to keep it up for 10 or 20 minutes to see if there's any trouble.

ETA: 11 breath cycles in 6 minutes, then I got bored. It's very deliberate breathing, filling my lungs as much as possible with each inhalation, but doesn't feel uncomfortable in any way.