Somewhere in the Accounting tab of the printing monitor dialogue it will tell you how much of each color of ink you have used. Based on that, I buy whatever size cartridge of each color that I think I can use up in 6 months, which is what Canon recommends for in-machine cartridge time.
Yup, Yellow is the 8300's favorite color! Normalized to yellow, here's what my machine has used so far.
Yellow 1.0
Gray 0.92
(Black) 0.75
Photo Magenta 0.73
Photo Cyan 0.59
Blue 0.55
Photo Gray 0.42
Cyan 0.40
Red 0.34
Green 0.25
Magenta 0.23
3.5 maintenance tanks and 3 heads for just over 20 liters of ink.
One of the head deaths was unnatural, due to an argument with an insect coated into some budget canvas. One of the maintenance tank changes was caused because I was in denial about needing a head change. And I think I use unusual amounts of Blue due to a couple mood-indigo photos that I sell a lot of.
Yes, I've thought about picking up a machine on sale as an economical way to restock ink, heads, and spare parts. However, that would be very uneconomical in terms of logistics, so I'm just gonna keep buying more carts and heads until either the 8300 dies, or the 9300 comes out. No mechanical failures so far other than knocking off and losing most of the stupid little v-shaped swingy-thingies, knock on wood.