Oxygen absorption depends on the density and temp of the wort. Higher gravity means less O2 absorption, so you'd need to run the O2 longer to get the same amount of absorption.
You're adding it to wort before pitching, so temp would be pretty constant, +/- a few degrees, batch to batch. If you're brewing 1.040-1.050-ish ales, then you'd probably only need 10 seconds of O2.
If you're making a lager or a high gravity beer, you'd need a lot more O2, and in those cases, I'd still probably go with 25-30 secs.