Bought these as my wardrobe smelled musty, After 2 or 3 weeks of having them in I found that they were removing water and that the musty smell was lessening. Now some 6 months later my wardrobe is musty free and I have re-ordered some more. Yes the old ones lasted that long before I considered they were finished.
I've had 2 of these hanging in a troubled wardrobe for a couple of weeks and each now has about a cupful of water in its reservoir. They will no doubt be full up in a couple more weeks. They definitely work, but in a really damp space you'll get through quite a few of them I think.