How long does gasoline last for use in lawnmowers?

over the course of several months, gasoline will gradually break down and lost its octane, which can easily cause your mower not to start. a good way to solve this is to add some fuel stablizer into your gas can(especially if you're cutting your gr for the last time before winter). this helps maintain the octane in the gas and you shouldn't have a problem starting the mower after a few months.