That very well might of been your experience,....but over the same time frame, a sodastream product will lose its fizz way faster than a commercially bottled product.
I just give it a few extra squirts of fizz (like 5 instead of 3). Also, make sure you tighten the cap down good after pouring to keep it fizzy longer. I can't say how long-term it is because a bottle usually disappears within a week, but regular soda will go flat over time too, so meh.
I haven't really found it to be as useful as I thought it would be. I like the concept, but you can only carbonate water, so if you want carbonated juice, you have to dilute it with the fizzy water. And if you're going to do that, why not just buy a big bottle of plain seltzer water for a dollar?
I'm also not crazy about most of the flavors; I think if they had partnered with Pepsi or something to offer official flavors that you already know & love (like Sprite) as kind of a novelty device, it would have been a lot better, but I don't want to have to buy & store bulk commercial syrup just to go the DIY route.