I think it depends on the height/width ratio of your wort inside the fermenter you're oxygenating. That column of wort, if shallow will dissolve less O2 than a deep one, as any bubbles that reach the surface won't oxygenate your wort anymore. When the way up is longer, more O2 can dissolve.
In that light, I use a flow rate of 1/4 l/m for 4 minutes in a 5.5 gallon batch in a standard brew bucket (6.5 gallon ale pail). I move/stir the wand around periodically, about every 30 seconds, to as new "corner."
So yes, for half a batch, oxygenate at about half the time or much better, half the rate (because the wort is likely shallower).
As
@Golddiggie said, it's tricky to regulate the output on those little red tanks. They should be good for 20-some 5 gallon batches for 1 minute each, only cracking them open just enough to see a coating of bubbles emerging from the stone, and rising. Some surface foaming is normal, but when you see any rippling or bubbling you're wasted O2, so close the valve down a bit.
I've heard those valves on the mini red tanks can sometimes leak, so maybe best remove the valve between uses.
If you're a serious brewer and brew enough, get a real tank. 40 cf may last nearly a lifetime.
Buy a regulator for around $30. Mine goes down to 1/32 l/m, which I use for oxygenating yeast starters, or leftover wort in a gallon jug.