I've had some for up to 8 months. DO NOT keep buying new cartridges!!! Just rinse the ones you have out thoroughly with a hose setting on "Mist".
This way, you get the nasty stuff out of the cartridge for the most part and keep some of the beneficial bacteria in the process.
It's been working for me for over 5yrs now. (When I started buying Emporers)
Hope that helps...