Having a lot of different inks is one of those fountain pen geek problems that most people don’t get.
Some people have different inks in different pens, while others just use the same in all of their pens. I usually have the same ink in all of my pens because then I can just pick up the closest pen in my cup when my current pen runs out of ink.
I try to use the same ink for longer periods. This is to avoid having many bottles that are almost empty. And I try to use up what I have of one colour before I move on to the next one.
For me, the same goes for notebooks and inks. I like to complete something before I move on to the next.
I obviously don’t use the whole bottle before I move on to the next. The exception is, as mentioned above, when I have two similar colours.
The way it works is that when I get something new I ink up my pens as they run out, and test out the ink with all of my pens for a while. And then I just rotate between what I have. I usually keep to one colour for 6-8 weeks.