There are differences of opinion about this. I personally have several cases of Dasani water in storage--dark and cool--from three or four years ago. Will they be good to drink if I go and crack one open right now? I'm pretty sure they would be, at least in terms of micro-organism growth, etc.
However--there may be some plastic-bottle chemical byproducts that have leeched into the water (i.e. BPA--which is a carcinogenic toxin).
To be safe, it's a good idea to plan on filtering water that has been stored beyond six months or so. Boiling or chemically purifying water that has stored for a long time also may be a worthwhile step to take if you are not rotating your water storage.