I've been using these as my main PV for about 3 months. They tend to vary on how long they last. Some lasted me 2 months, others only a week or two. Of course this depends on how much fluid you pump through it a day and other factors.
When these things start to go you'll notice a lack of performance, and many times you can visually see that the fiber mesh on the inside is loosening, maybe falling apart. The fiber can also get burnt, making the flavor taste bad. Some of these come out of the factory with loosely packed fiber, and I've noticed these don't work great from the get go.
Since these things tend to fade gradually, it can be tough to tell if they are starting to perform poorly, or if you're just going crazy. When I'm feeling that the cart isn't performing as it should, I usually prepare a fresh one and compare, and if there isn't much difference i table the new one until the next time I think it's going bad.
If I had to guess, putting 2-3 ml/day through one will last me roughly 2 weeks. It will still work, but performance will start to degrade.