For one you are using an air tight (metal) can, there is a reason that premixed fuel is sold in it and that is shelf life. Now i took chemistry in college and if I remember right octane loss will be minimal in a sealed container so it is a moot point. What you need to know is when you put the fuel in the can humidity in the air can make a difference, so i would fill the can to the brim and get all air out. Now you have another factor removed from the equation. The other thing that can make a difference is your additives. I like star tron, or amsoil fuel stabilizer a lot better than stabil brand, stabil is a fine product for general purpose storage, say like a month or two, but if you go and try to get years out of this stuff you need a high quality stabilizer that is focused on internal oxidation basically a chemical reaction blocker like the amsoil product or star tron. Also temp and how you store the can matter a lot, as for storage keep the can out of direct sunlight and try to keep the room at 50 to 70 degrees, do not put the can on a metal or concrete surface as they are good thermal conductors and will cause internal separation or layering (wooden shelving works great). A good practise would also be to shake it occasionally to keep a good mix of the product. If you follow all my advice a few years is not out of question for saws and longer for engines of lesser design (push mowers, etc).