I can get 2 weeks from a z coil, I've done the math as to how long they'll last yet I'll still probably get a few more boxes.
Just as an FYI: I use unflvored NIC and usually get 3 months out of a Z-coil. Also, you can rinse and/or clean them to get even more use out of them, especially using a Sonic Cleaner. . . BTW: Ya' can Rinse them well & save them and do them all at once in the Sonic Cleaner . . . Just Sayin' . . .
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or is there a rinse cycle?