There's a thread on here where we discussed this, and I ran some bottled wort through a dissolved O2 meter. I can't remember the exact numbers off the top of my head, but about 60s wide open for low-medium strength ales got me to about 10?ppm, and about 90-120s got me up to 15ppm for lagers. So I do 60s for regular ales, 90s for big boys, and 90-120 depending on the strength of the lager. Really big beers get the first dose and then another shot 8-12 hours later. If you can dig up the thread it's got a lot of good stuff in it.