I've got some cartos that I've used for the last 9-10 months. In fact, I've only ever had 2 cartos actually die on me.
I've got a couple of flavors I put into some cartos and didn't like them, so I capped them and put them in my case for just in case I like them later. Every month or so I'll pull some of these out and take a puff and realize why I didn't like them in the first place. But, they still vape and haven't dried out yet. I have a cuban cigar one that has been filled for 8 months or so and still has juice in it.
I don't use them all everyday, but I don't see any problems with mold. The amount of heat required to vaporize the liquids, as well as the PG base of the liquids, should prevent any kind of problems with bacteria/mold growth.
Of course, if it makes you feel uncomfortable vaping something which you suspect might have a change to promote bacteria/mold growth, you could always just switch to a brand new carto every day. It's not quite as cost effective, but you have to determine what exactly is your cost/comfort ratio.