This is something I have been mulling over also. I think there are several considerations:
(1) First, true consumables (things *intended* to be consumed or used up with use) usually have a shelf life. Whether it's tuna or batteries, they degrade with time and it doesn't make much sense to hold onto more than you can use in a reasonable amount of time. That will vary by the shelf life of the product.
(2) With consumables that are tied to another product (say, a vacuum cleaner bag, ink toner cartridge or an auto air filter) -- in my experience, at least half of the time the original appliance or whatever dies before I run out of supplies for it, and then I'm stuck with a crate of something for a thing I no longer own. I think that if I ever did a universal and complete accounting, I would have lost more money than I saved by stocking up on such things. I usually keep one or two spares, depending on how quickly they get used up and how long I anticipate the primary appliance will last. Generally, I keep one replacement on hand.
(3) With same consumer goods (say, your electric kettle) -- by the time mine breaks, there has usually been an advance in the technology and I can get something better at a cheaper price. Or the old thing may be obsolete -- like a VHS player, or a non-HD TV. Or I've changed something (The bathroom is now blue and I no longer want green bath towels.) Or there will be an even better sale.
(4) I think it also depends on how organized you are. You need to be able to FIND those things you bought and remember you have them. You also need the appropriate kind of storage space (perhaps temperature controlled, or at least not in the garage.)
I guess what I am saying is that five years from now when you need the new whatever, the one you have stockpiled may not be the one you really need or want. Meanwhile, you have to store it, tend to it, remember you have it ...
