It can take anywhere from hours to minutes, it all depends on how hard you drive. Beat on it like a red headed step child, or go to the drag strip, and it'll get hot quick.
Some have gotten ingenious, putting the heat exchanger in the trunk inside an ice bath, or using the A/C core as a chiller in front of the heat exchanger... All requiring a whole bunch of extra work and time, introduces potential problems, and makes using the car as a daily driver impossible or impractical.
It's all up to what you're doing to the car, with the car, and how much you're willing to spend.
