cooling wort

I have a 3 Sanke keg brew setup; HLT, mash tun and boil kettle. Straightforward. Brewing in Houston in the summer is a bear because it's so hot and the tap water is fairly useless for cooling unless you use huge quantities to get down to ~90 degrees. Well, we're building a home out in Hill...

5.4 gallons OG 1.064 aerated wort in sanitized fermenter. Cooled to 70°F and placed in a 45°F garage. How long before the wort reaches ~54°F just by being kept in cooler area?

So my question is not actually about the effectiveness of this: I pre-boiled about a half gallon of water, let it chill and poured it into a sanitized container, sealed it and freezed it. I plan to cool my wort by gently transferring the ice cylinder into my kettle at flameout (in a cold water...