It's a common question, and while the answer should be simple, there are actually a lot of variables and factors to look into before this can really be answered. Determining how long ink cartridges last depends on how often you print, what you print, what type of paper is being used and whether or not the ink is expired. This article will present all the factors you should consider when trying to answer this question.
The easiest factor to look into is how much ink is inside the cartridge. You normally do not have to go by millilitres or anything like that, but most cartridge packaging will state how many prints the cartridge can perform before being depleted. However, as most people find out, this number is normally not accurate. For example, a cartridge that is supposed to print 5,000 copies may only print 3,500. It isn't because the company is lying or inflating the print amount, it is just that other factors are more important than this.
One factor is how much of the paper you are printing on. Normally ink cartridges state the number of prints based on about 5-10% paper coverage. This can be difficult for even the mathematically adept to visualize, but it is normally how much ink is used for a common single-sided page of text. If you are printing around this much on each sheet, then the estimated number will normally be right. However, if you are printing images or printing a solid block of colour for the background on a sheet, this drastically decreases the total number of prints.
Another factor with ink usage is how much ink is used to saturate the paper. When you set something to print, you can normally select the quality level. If you have ever played with this, you may have noted that low-quality prints are light and high-quality prints are dark and vibrant. This is because quality dictates how much ink is used on the paper. Low-quality uses less ink, while high-quality uses more. While you may be tempted to print everything in low-quality to save on ink, the estimated ink number is for the normal quality, and this is normally best for most print jobs.
Along with the quality level, the paper used may also affect how much ink is used. Thicker papers, such as cardstock and photo papers, often need more ink when compared to regular paper and archival paper. Most printers have a weight setting along with a quality setting, and heavier weights act like higher qualities: they use more ink. You can set the weight to normal with a thicker sheet, but the prints may look poor.
Expired ink may also cause the cartridge to stop working, even if there is still ink in the cartridge. This may override all the other considerations, especially if the ink is very old. When the ink expires, it may dry up, meaning it cannot be printed with. Even if the ink is not expired, if it is not used for several months it may still dry up. In this case, it is normally best to print every two or three weeks, even if you do not need the print, just to keep the ink in good condition.