2 liters of soda everyday is huge. If it was sometimes (in a party, etc) it would be fine, but I can't imagine drinking close to that amount everyday.

I am sure that people do even worse, but at that point you're in a bad situation if you drink normal (e.g.: the amount of sugar) or diet soda.

2L of pretty much anything that isn't water is very likely too much.

Put it this way: go read up experiences on reddit of people doing the GOMAD diet.

2 litres of (diet) soda is definitely an absurd amount