See the info in Response 2.
The manufacturer often states that you must use up the ink in the cartridge within a specific time period once you start using it, e.g. a year, and that it should not sit unopened before you use it for longer than a specific time period, e.g. a year or two, before you use it, otherwise they don't guarantee the ink / the cartridge will work properly, to protect their butts.
The time period is probably based on a worst case scenario - e.g. the ink is exposed to higher than standard room temp. If you keep the not yet used cartridge in a cool place, and if where you have the printer is not hotter than normal room temp all or most of the time, the ink will be okay for longer than that. Also, I've found if the ink is dye based rather than pigment based, the ink will last a lot longer than that.
Pigment based inks may coagulate inside the cartridge after quite a long time, but other than that, the problem isn't that the ink inside the cartridge dries out, it's that the ink can dry out and clog the print head once it's in the print head.
The important thing is you should print so you use all the colors at least once a week or so.
If you don't think you'll always do that, get yourself a black and white or color laser printer - no clog worries - the toner cartridges cost more but the page yield is much higher, it may be years before you need to replace one, and they can be refilled locally, or you can get a kit to do that yourself..
The average number of pages rating a cartridge yields isn't necessarily defined the same way for different printer manufacturers, but it's similar for different cartridges for the same manufacturer's products. The rating always assumes you're not covering the whole page with ink, only a certain percentage.
The more ink in the cartridge, the higher the page yield, the less it will cost you in the long run.