mrs_o is quite right. Six months is fine.
Champagne does have a tendency to lose its fizz if you keep it for too long, and the fresh looking colour and the taste also suffer.
Sometimes, it will last for years, as mrs_o has found. But last Bank Holiday, we had some friends over and opened a bottle of 1962 vintage champagne. After one taste, we tipped the bottle out on the lawn. It was ghastly. Flat and sour.
Then we opened some young Prosecco, and it was wonderful. Vibrant, bubbly and fruity. Perfect for a warm afternoon (so not needed today then!!)