I have had no problems with it separating after sitting for weeks at a time... After a while the gasoline will go bad, especially if it is exposed to sunlight and heat. If/when it starts smelling funky, throw it out or put it in your car and mix a new batch.

When flying the two stroke engines in the Ultralight Airplanes, I had a personal rule that I wouldn't fly with gasoline that had been mixed with oil for more than two weeks. A lot of people fly gas that has been mixed for a month with no problems though. Once the gas has been mixed with oil, it slowly starts going bad however, and it's something to watch out for with any two cycle engine. But these little engines we are using for model airplanes are much less sensitive to octane, and if the gas doesn't smell bad, it should be fine to use.

AV8TOR