What is cold crashing?
When is it done?
How long does it take?
Best temperature to make it happen?
Why It's Done
When It's Done
Cooling Rate and Time
Cold crashing is a technique to get the yeast to flocculate (settle to the bottom of the fermenter). This is generally done to get clearer beer (or wine).
It should be done when fermentation is complete, since there will be very little (if any) fermentation activity afterwards. This is because you are effectively removing most of the yeast from the beer. This is not a reliable way to stabilize the beer, though, so I would not recommend thinking of it for that. You should still assume that there is active yeast in the beer, and if you were to add more fermentable sugars to the beer (e.g. to backsweeten or to prime it for bottle conditioning), fermentation may restart.
This process takes at least a couple of weeks, though the longer you do it, the more effective it will be.
Regular refrigerator temperatures (in the 30s F) should work pretty well.