well couple answers to that question and alot has to do with the conditions it is being stored in.....temperature cycle and range, humdity, above ground or below, alot varies. keep a biocide in it to keep out the critters, dont leave it open to air, sunlight, or moisture, minimize the temperature swings that it will have to endure, and it will last you a long time. i know more about the chemistry of biodiesel (which differs alot) but i can tell you that over time the carbon carbon bonds (the breaking of which under compression in the engine is what makes it a fuel source) start to weaken and the bond sites break, essentially lowering the centane rating of the fuel (read: less bang for the $3.30 a gallon).