Having had a vehicle that sat for about 9 months without running, I can say that what Cdnwolf mentions is pretty accurate. I had more problems with keeping the darn thing running to get it to a gas station than I did with there not being enough gas to do it. Water condensation, gas degradation and particulates all have a play in gas that's sat for a while. Sure, you might be able to strain gas of particulates, but you're still going to have water in the mix and gas degradation to contend with. It leads to a rough running engine.
So I'd say, depending on the humidity in the area and how "sealed" the tank really is, you've probably got 5 months or so before you start seeing a noticable difference in the performance of the gas. Evaporation probably wouldn't really take effect until years after no usage, but by then, the gas is so degraded and filled with water that it's useless.
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