Yesterday, I cooled 3 gallons from boiling to 74 degrees in my kitchen sink in 20 minutes. I dumped a 20lb bag of ice into the sink, added tap water to about halfway full, put the kettle in, then continued filling the sink until the water level was above the level of the wort in the kettle. This is key since you want to maximize the surface area that the cooling liquid is giving the kettle. Also, stirring the wort while cooling it makes a huge difference since you're increasing the amount of wort that can transfer heat to the water. I've got a deep sink, so that helps. When brewing at my friend's house, cooling takes about 15 minutes longer since he has a shallow sink.