You can extend the life of the replacement cartridges by rinsing them out frequently...you should be able to use them for up to a month and a half, after that the filter pad will be fine but the carbon will be depleated...also something I have done in the past, if you dont want to use the media basket...do this
Do this next time its time to change your cartridges if you want and dont need the extra carbon if you dont have a high bio load...
Take out your baskets and put your OLD cartridges in the back slots where the baskets would go after they have been rinsed out, and then put your NEW cartridges in the front slots where the old ones were, that way the old ones act as a sort of prefilter to get the goop before the water gets to the new cartridges!