Gasoline can sit in a cars gasoline tank for around 30 days before it starts to go bad and degrade.
After 30 days and beyond the gasoline gradually gets older and older and begins to lose it's octane rating as well as begins to turn stale.
Eventually after a year or two of the gasoline sitting in the cars gasoline tank the old gasoline turns to varnish which then requires dropping the gasoline tank and cleaning it or replacing the gasoline tank with a new one.
If you have gas older than a few months in the gasoline tank you should remove the old gasoline and put new fresh gasoline in the tank.
Otherwise you risk the new gasoline going bad too soon due to the older gasoline.
If you have gasoline in your tank you should use it up within 30 days by driving the car more often and then replenish the gasoline tank with fresh gasoline.
You should also add some HEET and other fuel additives to the gasoline to help extend the gasoline life but still you should be using the gasoline up within 30 days so that it doesn't go bad.