Not really all that long. Adding stabil will help though. Best idea if you need to keep gas around for power equipment is to bring the tank with you when you fill up your car every once in a while. Dump the can's fuel in the car and refill it, every so often so that you always have fresh fuel available.
I've stored full gas cans with no additives for over 6 months with no problem when I lived in WY. I used to top off my mower and motorcycle tanks also (to minimize condensation) and they sat for 6-8 months, then always started right up in the Spring. I work with a guy here is SC who is paranoid and puts Stabil in his gas gans, etc when winters here are pretty mild and short. So I don't really know, my experience is that at least six months storage doesn't seem to be a problem.
Ethanol additives to gas have complicated the issue of storing gas as it absorbs moisture. Stabil makes a new product that helps address this. It is aimed at the marine market where this is even more critical.
Three months is probably ok for untreated gas, but for longer, definitely treat it. Stabil is good for a year and probably more, especially if you double the treatment.
Winter gas has more of the additives that get lost during storage, so it will last a bit longer than summer gas. These are the chemicals that help with starting in cold weather.
There are a number of other gas stabilizers on the market, PRI-G is one, but I have only used Stabil for the gas I keep during hurricane season.
I keep gas in sealed cans for my generator for about a year using Stabil in the cans. Normally at the end of hurricane season I will use it in the car and refill the cans. No issues so far and I have been doing this for 6 years now.
In a motorcycle that hasn't ran for a couple of months, I'd drain the gas and put fresh stuff in. I would think gas sitting in a plastic jug would be okay for 6 months or so, but it would depend on what you'd be putting it into.