How long does unleaded petrol keep for?

I do a lot of driving and sometimes find myself in situations where my petrol tank is nearly empty and I’m not near a petrol station.

I have an opaque plastic petrol canister with a five litre capacity and a screw-top. I keep this filled with petrol in the boot of my car.

I know petrol deteriorates and that old petrol can damage some mechanisms inside cars. I’m also not likely to need to use the fuel in my canister very frequently, as it’s really just for emergency situations. How long should I keep this petrol for? I was planning on periodically putting the old canister fuel in my car and refilling it to make sure it is fresh and usable.