It all depends on the juice and how solid my build is. As has been mentioned, some darker juices need frequent replacement. I have a yellow cake juice I love, but it barely makes it through a tank without gunking up the coil, and consequently burning the wick. Not tasty, and inhaling burnt rayon is not good. It sucks, since I love the juice, just don't love re wicking constantly.
On the other hand, I have a bubblegum juice in one of my kayfuns that I haven't re wicked or re coiled in at least two weeks, maybe three, I really don't remember. It's a clear juice, and doesn't gunk up coils, so if my build is good, it just lasts. I have another with butterscotch that's probably a week and a half at least as well. It does need re wicking, though. Really I think the juice you're vaping, assuming your build is solid, is the biggest factor.