Gasoline goes stale after a bit of time in the tank or jug. No scientific knowledge of exactly how long but you can smell the difference between really stale fuel and good fuel. For most outboards, wouldn't keep fuel longer than a year. Running the engine with the fuel shut off so the carb is run dry really cuts down on problems with small engines. It's not the fuel going stale as evaporating and leaving varnish on the interior of the carb. Ethanol is miscible with water and will pick up water from the air in the tank. We we all know how well water burns. That's makes ethanol gasoline a particularly crappy fuel for a boat.