I'm not sure about coconut fiber, but sphagnum moss can last at least several weeks to longer, depending on whether or not your leo is defecating inside it. The moss is mold-resistant and keeps its moisture well. You only need to add new water a couple times a week.
The only downside is that it can make a mess if your leo likes to "dig" through it like one of mine does.
I know this sounds kind of wth-ish but, when it gets dirty. Every gecko is different. Some poop in their humid hides, some leave skin behind, sometimes you get a bad egg, theres a lot of factors that make a hide dirty. I go by two things: Can I see anything that is "dirty?" I use cocofiber so if theres a poop in there and its like in a corner I will often just change out that corner, not the whole hide. And second, does it smell "dirty?" If theres a moldy smell or a poopy smell or a rotton egg smell, then I change out the whole hide. Some of my geckos go for weeks with the same cocofiber because they are just really clean in their hides, some need it changed more frquently, so it really does depend. Hope this helped a little.