We go through a keg of soda water about every three days.
At the moment I have two gas bottles hooked up to a kegerator in the kitchen, one at 'normal' beer pressure and the other at 200 kpa for soda water.
As I wanted to keep two of the three available taps for beer but also a new keg of cold and carbed soda water ready when the old one blows I have an old bar fridge in the garage which cools and carbs soda water at 200 kpa. So basically two kegs constantly in rotation.
I use RO water, there is also some sort of 'alkaline remineralization' cartridge hooked up to give it a bit more life back.
I've tried the San Pellegrino recipe that's floating around the forum here a few times but we agree that the straight RO tastes much nicer. But I know others apparently had good success with it.
I'm currently experimenting with different salt additions to try and find a balance but haven't found anything that beats straight RO yet.
Anyway, soda water on tap is one of the best things I've ever done I reckon and the wife is forever grateful.