providing the fridge is working it should keep in there an eternity however it will only be worth drinking within six days and then deteriorating by the hour . but in most households it will have been drunk within the first hour
According to some (I've never tried it, so don't know), putting a spoon into the neck of the bottle, prolongs the fizz. Sounds like a lot of balloney to me.
I have a neat little thingy which clamps over the neck of the champagne bottle and seals the fizz in. Its called a champagne keeper and I got mine in tescos....in our house it gets hardly any use though!!
The spoon idea is indeed a myth. And don't forget to keep it in the actual fridge, and not in the door, as the constant opening and closing of the fridge door will lower the champagne's temperature, causing the bubbles to escape. Who wants flat champagne? Nobody. Nobody at all.