Coleman fuel (also known as white gas) is, chemically speaking, very close to the gasoline you buy at the pump, without all the additives your car likes and the government requires, so storing it provides the same difficulties and dangers of storing a can of gas for a lawnmower.

Most estimates I've seen say that it starts to go bad after a year. That said, backpacking stoves are less complicated than engines, and if you know how to clean and maintain your stove you can burn white gas that is much older. My record is five years with no clogging, and only a slight drop-off in performance. Of course, I have no idea how long the stuff had been in the can when I bought it. I wonder if there's a date on the can . . .

I used to buy the gallon cans of Coleman fuel, but have recently switched to the ultra-pure stuff from MSR (http://www.msrgear.com/stoves/superfuel.asp). It's much more expensive, but white gas stoves are so efficient that the cost is still minor. Smaller cans equal less waste, and the higher purity makes my stove a real rocket. I can tell by the sound which type of fuel I'm burning, and I get faster boils using less fuel.