The perfect keg temperature is 3 degrees. You will have to chill the keg for 8 to 12 hours from a warm state. The cooler the keg before you put it in the machine, the faster it will get to the perfect temperature.  A common thumb rule is to cool the keg for an hour to get a degree of cooling. You can go pre-chilling the kegs in the fridge or the coolest place in your home if you do not want to wait that long.