as long as its putting out vapor its ok.
but buildup on the coil will lesson the popping/cracking.
i used to soak them in a cup of hot water or alcohol and rotate them during the week. as with all things disposable after a while they just dont work right anymore and it's time for new ones.
i have seen them last from a few days to a few weeks and some of it will depend on the
juice and how sticky or built up on the coil it gets.