Champagne always tastes best well chilled, but the exact temperature for chilling is a matter of personal taste.  Three to four hours in the refrigerator is fine, or if you are caught unprepared, a 30 minute blast in the freezer will do the trick.  A word of cautionary advice; don’t forget about the bottle in the freezer – if the bubbles freeze, the cork will shoot out and quite possibly the bottle will explode – I know, I’ve done it!  If you are using an ice bucket, ice mixed with water will cool the bubbles much faster than ice alone.  Chilling Champagne helps reduce the pressure inside the bottle so that it is easier and safer to open, and there’s no wastage.