Please read the article below if you'd like to make them last a lot longer Almost forever.
Inks are water solutions and cartridges are made of what seems to be polyethylene. Over a period of years, water molecules will migrate through the walls of the cartridge--it has nothing to do with whether the cartridge is sealed properly or not--it will happen anyway. This situation is roughly analogous to a balloon filled with air. After several days, air leaks through the rubber, no matter how well it is sealed. The bottom line is that cartridges have a shelf-life on the order of years. They should be fine for about 3 years or so at room temperature. They can last much longer at lower temperatures, but don't allow them to freeze--the water will expand and possibly break the cartridge. In theory, a way to make cartridges last *almost* indefinitely, would be to store them in a jar filled with water. This would *essentially* keep the water inside the cartridge in equilibrium with the water outside--i.e. about as much water would leak in as would leak out. The reason that I said *almost* is that there is a very weak force of osmosis at work. This is because there is a difference in the concentration of molecules dissolved inside the cartridge (i.e. the ink and small amounts of other additives) and outside, where there is nothing dissolved). This force will very slowly pull water into the cartridge as the exact concentrations attempt to reach equilibrium (this is the principal of dialysis). However, we are now talking about a process that would take perhaps decades. Also in theory, if the water in the bottle had the exact concentration of large molecules as inside the cartridge, the cartridge would last more than a lifetime. Chemistry in (very slow) action.