Hi windy, without knowing what kind of equipment you are using, here's what I can tell you.
You will notice a decrease in
vape output. You will also be able to tell that the polyfill is dry at least on top of the carto. You might also notice what is called a "dry hit" which taste like a bit of a scratchy feeling in the back of your
throat.
I've moved on to
tanks now, but back when I was using kr808 cartos, it seems like I added 5-8 drops every hour or so. This is going to get easier as time goes by. You won't think about it, you'll just sorta know.