I've been using a 2 micron stone, a medical regulator set at 3 LPM for 30-60 seconds, depending on type, gravity, etc. According to your formula,
time = (0.00265 * ppm * volume) / flowrate
if I want 10ppm, for 5 gallons, at 3 LPM i'd flow for .044 minutes, or 2.65 seconds. That can't be right.