Yes, it probably won’t be completely flat after half a liter but the fizzyness will be noticeably weaker and will only get worse the more you drink, especially if you don’t get to it soon. Two liters are ok if you’re sharing them with multiple people who will finish it right away or the next day but they suck for one person unless you drink a ridiculous amount of soda.
Are you screwing the cap on all the way?
Yes, it probably won’t be completely flat after half a liter but the fizzyness will be noticeably weaker and will only get worse the more you drink, especially if you don’t get to it soon. Two liters are ok if you’re sharing them with multiple people who will finish it right away or the next day but they suck for one person unless you drink a ridiculous amount of soda.
I think you’re just shaking it too much. If you are gentle to avoid losing the carbonation and put the cap on tight, it can last for days.