OK, some folks posting here clearly didn't read the OP's question or operating conditions while others have posted helpful feedback. Let's hope this response falls into the category of helpful feedback.
As others have mentioned, oxygenated fuels (that translates to gas with alcohol) have a short shelf life. We've already seen comments about the alcohol potentially absorbing moisture and potentially losing octane. Bottom line is that alcohol-gas mixtures don't last a long time in the tank, especially when the tank is vented to the atmosphere as our aircraft tanks are.
Pure gas doesn't suffer the same rate of deterioration as oxygenated gas. I have been burning pure mogas for about 16 years now in one of my aircraft. Through these years of experience I have learned a few things.
My chief point of learning is that pure gas in a sealed container (jerry can) will keep for six months without much change in performance characteristics. Put that same gas in a vented aircraft fuel tank and it will change characteristics perceptible to the human. It's odour changes, it's colour darkens. How long this takes is directly impacted by temperature - the hotter it is, the faster the gas changes.
Through experience I have learned that leaving gas in the tank from late spring through mid-summer produced no change in operation of the aircraft. This aircraft is carb'd with no fuel pump on a gravity-fed fuel system - very simple.
I've also learned that gas left through an entire summer is not going to smell fresh and will be somewhat yellow in colour. Again, from my experience, this gas burned well.
A key factor in storage conditions is accessibility of outside air. A full tank is much less likely to degrade than a half tank. If the tank is only half full, fill it up with fresh gas so you at least have a 50/50 mixture of old and new. Go fly.
Another anecdote which reinforces the above statements... We recently have been lashed by a nasty storm which took out electrical power for a large portion of our city. My gas-powered generator sits in the uninsulated garage year-round. Its tank is full of pure 91 gas without any inhibitor or other additive. In this case the gas in the generator was at least a year old. While a generator may not be as critical an application as an aircraft engine, the truth cannot be denied. The generator burned that year old gas without any trouble. Keeping the tank full and the carb drained are two "must do" acts to preserve functionality in longer-term storage.