Most gasoline fuels are blended for an average shelf life of three months. Ethanol attracts water and that does not help, so the humidity in your area can have an effect, YMMV. Stabil works pretty well. I have found that old fuel gets a varnish smell to it, and at that point, I would drain it. Setting a car up for storage means draining all fuel or at the least, adding a stabilizer and topping off the tank so there is no room for condensation to develop.