Treat champagne as you would any still wine and store the bottles on their sides somewhere cool and dark. Doesn't have to be a cellar, but try and avoid the kitchen if you can where temperature fluctuates and strong cooking smells can eventually permeate the cork. If the champagne is still in a box of twelve, leave as it is as the bottles will already be on their sides. Champagne will keep as long as you want it to. Some folk like their champagne young, zesty and vibrant others like it older when it becomes more soft, mellow and buttery. In this respect champagne is like white burgundy (with which is shares a grape - Chardonnay) in that some like it young and some like it old. I know wine lovers who buy NV (non-vintage) champagne to keep for a couple of years before drinking as they like the added maturity. I had the good fortune to taste an extreme example of mature vintage champagne not that long ago when I tried some 1914 Pol Roger which was remarkable stuff, beautifully honeyed and creamy and full of nutty apricot flavours. Being so old the fizz didn't last long in the glass, sadly, but once we tried the old trick of adding a splash of a more recent vintage to the glass (in this case Pol Roger 1988) it fizzed again heroically without losing its own inimitable flavour.