I have a SodaStream which shoots CO2 into the water. I don't add anything to the water usually (nor baking soda either). I have 2 half litre SodaStream bottles (filled with plain water which I keep in the fridge). I find that I can drink half litre in one go; more and the bottle would sit around until the fizz unfizzed.
Mine is a SodaStream Genesis which I bought on eBay. Refils cost about $15 from Staples (return old canisters).