There are so many different varibles that influence how much ink your printer will use so unfortunately there is no one answer to this question. Because of the way inkjet printers mix tones, the total ink usage varies vastly with the nature of the images you are printing.
If you're printing high key images, then you will use far less ink than very saturated, dark images. Page coverage of course also makes a difference, as does how clean you keep your printer, and what sort of paper you're using.
Here's a decent rule of thumb - on average, all things considered including head cleans etc., we find that between 0.5ml and 1.2ml is used per full coverage A4 page, on fine art matte paper. That's total ink used for the print, cumulatively, from all your ink channels (i.e. NOT per ink channel).
This is a reasonable (but rather wide!) guesstimate. To put this in context, this means a complete set of cartridges for a printer like the Epson P800 will result in about 600 to 1200 A4 prints depending on what the actual images are. Being any more accurate than that is pretty much impossible unless you know exactly what you're printing and you're printing that same thing over and over again.
When planning ink usage here, we conservatively use 1ml per A4 in our calculations but very often find that is significantly higher than the amount we actually end up using.