Shelf life is very dependent on your sanitation practices, your storage, and your ingredients.
If you are meticulously clean, store your soda away from light in properly sealed glass bottles, force carbonate instead of ferment and include some preservatives or pasteurize, then you can probably get 6-8 months.
If you're nonchalant about cleanliness, store your soda somewhere where light hits it, use ingredients susceptible to spoilage, or ferment at all, then you're probably looking at a week or two, tops.
If you want to store large batches, your best bet is to hot fill (~160°F) a keg with seals that are stable at that temperature, pressurize it before it can pull a vacuum, then force carbonate. If you do that properly, then it's just like canning.