First, be certain that your beer is completely done fermenting. Use your hydrometer to test at least twice a couple days apart to verify that the gravity is no longer changing and is at or near the expected FG.
Next define what you intend to accomplish by cold conditioning. It you are mostly intending to clear up the beer, then as near freezing as you can get it for a couple days will be enough to settle out the suspended yeast and trub. Then bottling or kegging will be then next step.
If you intend to just let the beer sit at cellaring temps to condition, then longer is better, as in, months. The reactions that make beer "mature" happen faster at room temp so your beer may condition faster that way.