I wear makeup nearly every day, albeit fairly minimal - generally mineral foundation, curl my eyelashes, put on some mascara, neaten up my eyebrows, then some blusher and clear lip balm.
I've been using the same pot of No7 mineral blusher for almost four bloody years, even though I'm using it nearly every day. I'm very tightfisted on a bit of a budget, so I can't bear to just throw it out when it still does the job, but I can't help thinking that no one wants to be wearing the exact same makeup every day for four years. I do put enough on so that you can see a glow on my cheeks and it gives me a bit of life, but I'm not painting on massive layers of pink 
If it was an eye product I would inevitably have to keep throwing it out and starting again, as I would be afraid of eye infections - I replace my masacara and eyeshadows regularly for this reason.
Is it a big ploy by the cosmetics companies? If they sell a bigger pot, you have to pay more for it, but you won't get to the bottom of it before it goes 'off', so you go back to the shop and buy a big pot all over again?