Boy...........that's a tough question to answer. I've had the Oasis for about 11 months and have refilled the cartridge twice with distilled water. BTW, DO NOT OVERFILL as it can cause the unit to malfunction (personal experience speaking). I have ordered another cartridge refill to have on standby. One thing I have noticed as I've refilled the original cartridge: It seems to last for less and less time with each refill. I also wonder if the anti-mold agent is diminished each time? I figure I will fill it one more time (4, original plus 3 refills), then replace the cartidrige.
I've also noticed that they have come out with the Oasis Plus. The only difference I can see is that the Plus is "designed" to simply be refilled with distilled water. Makes me wish I hadn't invested in the original. Oh well..........Just my thoughts..............