Modern magazines have no problem with being loaded for a very long time, but I personally leave one round out because the magazine needs a little bit of play left in it for racking the slide over the top round.
Keep your ammo away from excessive moisture and humidity, don't let gun oil or other liquids get onto the primers. Ammo that you store long term without carrying it, try to keep it at room temperature and in a relatively dry and well ventilated environment. I'd keep them in their original boxes facing primer-up in the plastic trays they come in. Your carry ammo(if you carry) might need to be replaced once every couple months depending on the humidity it's exposed to and how often you chamber/unchamber each round.(if it's a semi-auto) If you chamber an individual round too many times it can push the bullet rearward in its casing which would cause too much pressure and a very dangerous situation when it's fired. This is called "bullet setback" and you'll be able to tell with the naked eye by setting the suspected cartridge next to a brand new one that's never been chambered.