Three inks for a very long time.
I have probably used Iroshizuku tsuki-yo in all of my pens that are in regular rotation for a little bit over a month by the time this goes live. I have used three inks except for some minor reviewing here and there since February 2016.
First I got the tsuki-yo, and I used that for about a month, before I switched to shin-ryoku, and used that until January 2016, when I got a bottle of fuyu-gaki and I used it until I ran out in the end of April, then I used the rest of the shin-ryoku bottle before I got back to my tsuki-yo.
I’m not that into switching inks all the time. The reason is that doing a proper clean is a pain in the ass to do that often. So I usually end up using at least half a bottle before I move on to the next thing.
Three excellent inks, but the only one of the three that I know for certain that I will re-purchase is the tsuki-yo. The shading is much more interesting, and it is kind of like black, as in that you never get bored by it.