If I used a steal drum, how long could I keep gasoline in there and still have it usable in a car, lawn mower, etc?
Also, how long could I keep it in plastic before it ate through the container?
Many plastics will hold gasoline indefinitely without the gasoline damaging the plastic. The main issue is the volume of the gasoline you will be storing. Anytime you get beyond the few gallons you might store in a shed you are dealing with a signficant safety issue, and there might be municipal regualtions limiting these amounts in residential neighborhoods.
IMO the dollar a gallon you might save if the cost goes back up is not worth the risk and investment in a tank and pump.
Here’s an article from the nice folks at Chevron.