When stored in a refrigerator, an open bottle of champagne can last between 3 to 5 days if it is re-corked or covered properly. If one does not have the cork or a stopper to close the bottle, then a piece of plastic wrap and rubber band can be used so that the champagne does not go flat before the specified time frame.
A common old wives' tale states that placing a silver spoon in the champagne before storing it in the refrigerator will keep it from going flat. However, there is no real evidence to support that the method works any better than re-corking.
An unopened champagne bottle can have a shelf life of 3 to 4 years, while older "vintage" bottles can sometimes last 20 years or longer. In order to maximize the shelf life, it is imperative that one follow proper storage directions. These include storing the bottle in an area away from direct sunlight, at a temperature of between 40 and 60 degrees.