The general suggestion for how much to keep in your stockpile is to have enough of each item to last your family between 3 and 6 months. This is because sales and coupons are cyclical. You don't need to buy 300 containers of mustard because within 3 months or less it will be on sale and have a coupon again.
Obviously what my family uses and what your family uses will be different in terms of products and quantity, so you have to figure this part our for yourself. For me, this translates to a quantity of between 4 - 8 of most of the items in my stockpile.
The time to break this rule is on items that have a long shelf-life, when there is a really great price or the item is free. At that point you can stock up between 6-12 months worth. For example, I have a year's worth of items such as hair dye, shampoo, toothpaste, deodorant, razor blades, and laundry detergent. Most were free - how could I resist? These items will not last indefinitely so I didn't purchase more than we'd use in one year.
There are other items that will last a long, long time, so I will buy as many of those items as I can if they are cheap or free: tissues, toilet paper, paper towels, q-tips, ziploc bags, and printer paper.