Like HexKrak said, too many variables. I'm in Dallas, too, and the one thing I will guarantee is that you won't get down to pitching temp in the summer using tap water. I have a 50 foot, 3/8 inch immersion chiller - I run tap water until I get down to about 80ºF in winter and about 100ºF in summer, then disconnect from the tap and begin recirculating ice water from my 10 gallon RubberMaid cooler mash tun. I pump it from the cooler through the chiller and back into the cooler and usually put about 7-8 gallons of ice water in the cooler in the summer, less in winter. You really need to either stir contantly (slowly but steadily) during the cooling or build a whilrpool IC which pumps wort back into the kettle tangential to the kettle wall to keep the wort swirling. I haven't timed how long it takes me, but I get down to 110-120ºF pretty quickly (maybe 10 minutes or so), but then it really slows down. Getting all the way to pitch temp. from 100 - 110 probably adds another 20-25 minutes (I need a bigger pump). After I reach pitch temp, I stir (whirlpool) for a few minutes, then let it sit covered for 20-30 minutes to let the trub settle while I sanitize the carboy and aeration equipment. I haven't had any problems with this time period.
