That depends on a variety of factors, such as heat, humidity, and other conditions of your generator’s storage. Moreover, it’s difficult to know how old the gas was when you bought it. It could have been fresh from the refinery, or already a few weeks old. Gasoline can start to deteriorate in as little as 30 days. As a rule of thumb, do not leave gasoline in your generator’s fuel tank for more than a couple of weeks, or make sure to use a fuel stabilizer.