Good question and Potter's response hit the ball out of the park...

Most of us can keep up with a flow rate of 40 to 45 liters/minute for as long as it takes to abort the attack...  Keeping up means you're able to keep the reservoir bag from filling tightly and have oxygen leak out while not being able to suck the bag flat as Potter says...

The best indication you've got a high enough flow rate is to increase the flow until you can't keep up then back it down a half turn or so on the regulator so you can sustain the respiration rate and depth of tidal volume...

That should have you breathing at or near vital capacity...(exhaling as completely as possible with an abdominal crunch and a single coughing maneuver to squeeze as much breath out as possible then inhale with your shoulders back as deeply and as rapidly as possible...) 

If you stand or sit erect and repeat this rapidly once every 3 to 4 seconds, you'll be hyperventilating deep enough and should start feeling the effects and symptoms of paresthesia, a slight tingling of the finger tips, lips, nose and back of the neck...  Most folks also feel a sight dizziness... 

When you feel these sensations, you've hyperventilated your way into respiratory alkalosis, and that is the fastest way to achieve an abort with oxygen therapy.  It's also very safe and far less invasive than the other cluster headache medications

Hope this helps...

Take care,

V/R, Batch