Gasoline goes "bad" in a hurry. This is a constant problem for anyone with a trailer boat, especially over winter. Sta-Bil will help extend the life, but it still goes bad sooner than you would think. Put bad gas in your engine, and say hello to a carb rebuild at the least. If you are going to store gas for emergency situations be sure to cycle it out on a regular basis, run it in your vehicle before it goes bad, and refill the storage cans. As for how long it takes to go bad, I try and not let it sit for longer than a month even with Sta-Bil.