Base on this: http://www.wyeastlab.com/hb_oxygenation.cfm & experience I have never over oxygenated with the 2 min mark.

We use one like this http://morebeer.com/view_product/16604/beerwinecoffee/Oxygenation_Partial_System the tanks run about $5 at hardware store.
2 minutes with the regulator about 1/2 - 3/4 open...

We noticed a big difference in kicking off by using an oxygenation stone & one of those disposable oxygen bottles from hardware store.