I usually drop the temp to -0.5 C in the fermenter with a balloon of co2 attached instead of the airlock to minimise oxidation. It takes approx a day to drop from 20 C. I leave it for 24 - 48 hrs at -0.5 C then transfer to corny keg and carbonate. More haze forming matter comes out of solution at lower temps, I once went lower than 0.5C and the beer started to freeze so haven't gone lower recently. I sometimes fine with gelatin ( especially lagers, not pale ales though) and beers come out pretty much clear in a week.