If its in a metal container kept in a constant temperature its good for a very long time but in plastic containers you can start to lose some of the vapors thru the plastic lowering its octane and if its in a hot then cold back to hot situation then it can suck in air along with moisture, so a couple months would be a long time.
I have cheated and added toluene and/or xylene to the container to bump the octane back up, normally about 6-8 oz to a 5 gallon can
