I try to keep a year's worth of ammo on hand for each weapon. So, I usually have anywhere from 3 months to 18 months on hand.
This doesn't have anything to do with zombies, civil disorder, nuclear war or any other emergency. It has more to do with having enough time to hunt for bargains and buying enough at one time to get better prices.
It's all in a locked closet. In our last house, where my wife had made inroads into every closet in the house, I tried hiding it in small lots around the house. I must have done a really good job because I could never find all of it until we moved.