That depends... In most cases it can last a long long time...
But, with a bit of moisture and some air it will grow algae (not actually algae, but a similar microbe) and will basically have a nice pile of goo growing on the top of it!
But, if kept in a good sealed container, no air/water it will last forever. At one of our jobs we were at this summer we were burning up some high sulphur diesel that has been sitting in a tank farm for about 30 years. No problem with it at all as the tanks were full and fully sealed.
Ive had some fuel in our yard that has been in vented containers and whatnot, within a few months it was growing crap in it already. but even then, you can get chemicals to kill off the microbes but that creates a whole new problem in itself. All the dead microbes fall to the bottom of the tank and you will be replacing filters for a while!
Its worse in the summer time as the microbes as most other living things tend to thrive in heat. so like any type of fuel, it should be stored in a full container, sealed, in a dark cool place.
