About a minute, give or take. Depends on the gravity of the wort. ETA: That's with hardware store cylinders and a 0.5 micron sintered stainless steel stone.
I do 45-60 for normal gravity beers, and once I get over 1.075, I go longer. 90 seconds usually. 1.100+ I hit it for 90-120 seconds, and then 45 seconds around 8-10 hours into fermentation.
The O2 calc in BrewCipher has served me well. I do also hit really big beers twice. Your mileage may vary if you use a different sized stone though.
