This is what Chevron has to say on the subject and this makes good sense.
22. How long can I store gasoline without it going bad?
Gasoline stored in a tightly closed container in a cool place will stay good for at least one year. It is better if the container or fuel tank is almost (95 percent) full. If the container or fuel tank will be in the direct sun or will be heated above 30°C (80°F) much of the time, add an aftermarket fuel stabilizer to the gasoline when you first buy it. Gasoline-oil blends for two-stroke cycle engines stored under the proper conditions will keep as well as gasoline itself. http://www.chevron.com/products/prodserv/fuels/bulletin/motorgas/8_q-a/#22
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