Welcome to the forum Mike!
You are correct, that is a tough question to answer. It depends on your vaping habits as well as what juice you use. I use juices that work great in cartos and can get about a month out of one, 2 if I clean it after a month. Now, that is best case scenario. I've burned a few and sometimes they get clogged and I have to toss them before the month. Also, I'm a very light vaper (1-3ml/day) so that's roughly 30-60ml through one before it's completely dead. I've heard most people report 30-45ml in the best case scenario. For some people, that can be 3 days to a week.
Let us know if you have any more questions
Glad to hear you are liking your kit!