Both good suggestions. Thanks!
I guess my concern is over-oxygenating - if that' even possible???
For all practical purposes, probably not.
I've always oxygenated 1 min with ale strains, 2 min for lagers, high-gravity worts, or O2-hungry ale strains. Am I doing it correctly? Well, I can't measure how much O2 is dissolving, so I've just let how the beers turn out tell me if I'm on track. After hundreds of batches doing it this way, yes, I am doing it correctly.
You just want a fine mist of bubbles coming from the stone. You're not trying to make the wort look like a hot tub. Any bubbles reaching the surface is wasted, undissolved O2. Gently move the wand around for maximum mixing.
With my last O2 tank (the small red welding kind), I got 155 minutes of oxygenation. That's a lot of batches. YMMV but those tanks last a long time if you don't waste the O2.