In the fuel tank of a car, motorbike or mower, there is an air-vent and petrol gradually evaporates. After months or years, the lighter fractions have gone, leaving only the heavier ones, and the remaining petrol is more like paraffin (I realised this when my stored car would not start...).
In a sealed can it cannot happen -- all fractions remain inside and the petrol should "keep" as long as the can lasts. Beware, however, of cans with a poor seal, which will vent under the high pressure generated in warm conditions. Incidentally these are even more likely to go up in flames.