How Much?

How much stock do I need..? I see maybe three factors to consider. First, what scenario am I planning for which will suggest a duration you intend to be prepared for. Second, what's my use frequency of the item I'm thinking about? if it's water, probably multiple times per day, is it keeping the brakes working on a car by having extra brake pads? Probably once every couple years. Third, would be do I have any other items that could be a substitute for the item I'm thinking about? If I'm stocking rice, but another grain would do instead, that might effect how much I want to keep. In general:

Prep Duration x use frequency x substitution factor = how much I need to keep on hand