Humidity is another factor. Water will cause bacteria and algae to grow in the fuel.
Other than that, it's failrly stable.
I went over to Riverside, CA in January, 2004 to pick up "Trick Truck" (see sig) after buying it from my cousin.
The truck had been setting since July, 2001 - 2 1/2 years. I changed the fuel filter as a matter of principle. After purging the air from the system, it fired up and purred like a CAT. There was nothing wrong with the 2 1/2 year-old fuel.