Cleanliness is next to Godliness. A clean(sanitized) tank with clorinated water will last a long, long time. I once traveled in a vessel and we had never changed the water in the tank since it was filled. As stated, the water doesn't go bad, things may grow in the tank and the gasses in the water will gas off giving it a old, musty taste but it can be very safe for years.
Our emergency water storage at home is in 55 gallon blue food grade drums with a couple of tablespoons of chlorine added and we usually wait three or four years to change. When we do change the water, we use it for irrigation around the house.