In this question defining cold crashing, there is some discussion of ramping the temperature down slowly over a long time, but then says quicker cold crashing is possible if the beer has already completed fermentation.
In this question, I'd like to explore an appropriate, quick, and effective cold crash technique on an ale that's had plenty of time to ferment and "clean-up". And for this example, the purpose of cold crashing is primarily for settling.
Given the carboy is in a refrigerator which is turned on normally (no external temperature control), it will take some time for the heat to be removed from the volume of beer. If the beer started at, say, 68F, with the refrigerator "on", there would be a ramp down, and within 6 hours, the beer would be at, say, 40F. Then you'd wait Y hours as settling continued.
What range of Y would you expect 50% of the eventual settling to be done? What range of Z would you expect 90% of the settling to be done?
Note that in this diagram, I have assumed that settling happens faster earlier, and slower later. This may or may not be your experience (could be linear). The blue line is the temperature of the beer. The black line is the amount of settling over time, starting at no cold crash settling and ending when 90% of the eventual possible settling is done.