I've heard of sodas lasting longer.....but when I worked for Coca Cola, we were told plastic bottles, 2, 3 liter etc, lasted about six months. We were shown bottles that sat for 6 months and a bit longer and the ingredients had separated.
There was a whitish sediment, sometimes fuzzy looking at the bottom. As a route merchandiser....they hammered us about being sure to rotate and front stock each day on our routes. Cans? Eh....cool keeping, I think they told us several months. Refrigerated, longer still. Again, they'll go flat fast if they get warm.
I have cans here that were bought a couple days ago, and the dates on them are November of this year. Carbonated drinks also have a habit of going flat. Take a can of Coke, Dr Pepper, etc, and tap it on a table a few times after you open it, and see how fast it goes flat.
One thing that used to be, and I'm sure still is....Sodas in a store, at least Coke products, are pretty fresh...like a couple days on cans. Maybe a few more for bottles, unless the store has them on ad and they're rotated quickly from the back. Large grocers get product in daily, so it nevers sits in the back long, if the grocery manager and the soda salesman are doing the job right.
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Most sodas now are sweetened with corn syrup. A few are bringing back sugars. As an aside...ever pop open a can of Pepsi, and get a metallic like taste? lol Not sure if it was the treated lining in the can or what. We used to joke about it. As far as diet sodas....mostly sweetened with aspartame, and usually don't last as long as reg. sodas.. Do a Google on aspartame.
Something to keep in mind....drinking a 12 ounce can of Coke is 39 grams of sugar. Dr Pepper I think, is 40 grams. I think root beers are more. That's the equivalent of just eating 8 or 9 teaspoons of sugar.
I don't believe sodas are a good long term stock item.