I suppose that depends on how much you print. I print commercially and although that doesn't mean I'm running hundreds (or even tens) of feet of paper through the printer daily, it was about 6 months before I started replacing the initial 330 ML carts. After 9 months, I have replaced all of them. When I get the low ink warning, I still can run a week or two on the cartridge, so I don't keep anything in stock. Delivery time is two days for my supplies. That way, I know the ink is always fresh. Not that you will necessarily have problems with older cartridges....