Unfortunately this one is a difficult question to answer. Factors such as the cartridge size, how much you write, your nib size and also your paper type all play a part. Cartridge size - some brands such as Parker and Lamy have larger ink cartridges.  These can last upto 50% longer than other cartridges as they hold more ink. Your writing volume - obviously if you write for longer periods and your pen usage is higher, you will use more ink and the cartridge will not last so long. Even as short as a few weeks if you are a writer. Nib size - A wider nib deposits more ink onto the writing surface and can therefore vastly reduce usage time.  Look out for this as often it is not even considered. Paper type - Some papers are slightly more absorbent and they can absorb the ink quicker as you write, which means you will be using more ink.